feedback5: merge W5AD — language packs as the first type of a generic content pack: a language is a file in the language folder (public/i18n/<code>.json with a _meta manifest, optional data/i18n/<code>.json) or a pack folder (packs/<id>/pack.json); the server lists them live at /packs/index.json, the menu shows every pack, strings fall back pack → base → declared fallback → Chinese, broken placeholders are skipped, plurals follow Intl.PluralRules (zh / en unchanged); tools i18n.mjs template / check, packs.mjs list / index, build-i18n --lang ja|ko|zh-TW; the release zips ship packs/ and packs/index.json; docs/PACKS.md, docs/I18N.md §5; §25.16 for now (renumbered next); golden: unchanged vs feedback5; vs W5AD exactly what feedback5 changed since f1df0d3 (CB5's 18 bot matches, CB2's roster 13 / bonds 7 / fields 6, CB4's roster-016 / 7 boss fields / standin-04/06/08 / diy-028/124)

No conflicts outside docs/history/0.2.0.md; the §25 intro keeps one sentence per stream (W5AD's line that also named
the hotkeys' §25.15 joined to CB4's).

Gate: non-browser suites 4981 / 4981 (test/i18n-packs 12 new, test/package 1 new), npm run golden ok,
test/golden.test.js 9 / 9, lint 0 errors, check:imports ok, typecheck ok, docs-consistency + docs-paths 31 / 31.

shared/i18n.js

@@ -2,51 +2,157 @@
 //
 // The Chinese source string is the message id: `t('整备区已满')`, `t('还剩 {n} 秒', { n })`. A message without a
 // translation in the current language falls back to the Chinese text, so an untranslated string is never blank.
-// Chinese ('zh') is the default language; English ('en') is a switch (the owner's decision of 2026-10-05: an English
-// browser still opens in Chinese until the player picks English). docs/I18N.md explains how to add and translate
-// strings.
+// Chinese ('zh') is the default language; every other language is a pack (shared/i18nPacks.js): a file in
+// public/i18n/ (or a pack folder, shared/packs.js) that the client lists from /packs/index.json and loads on demand
+// (ui/lang.js). Nothing here knows a language by name (the owner's decisions of 2026-10-05 and 2026-10-07: Chinese by
+// default, a language is a file in the language folder). docs/I18N.md explains how to add and translate strings and how
+// to add a language.
+//
+// Fallback chain per string: the chosen pack → its base (pt-BR → pt; zh-TW has none) → the packs its `_meta.fallback`
+// names (e.g. English) → the Chinese msgid (i18nPacks.js computeChain). A partial pack is fine: what it lacks falls
+// through. A translation that drops a placeholder of its msgid, or uses one the caller does not pass, or has a broken
+// plural form, is skipped like a missing one (setI18nWarn reports it; the client warns in development).
 //
 // Context: the same Chinese text may need two translations ('关闭' = "Close" on a button, "Off" on a switch):
-// `tc('toggle', '关闭')` looks up the key 'toggle::关闭' first, then '关闭'; Chinese shows '关闭'.
-// Module-level tables are evaluated once, before a language can change: mark their strings N_('…') (a no-op that
+// `tc('toggle', '关闭')` looks up the key 'toggle::关闭' first, then '关闭' (in each language of the chain); Chinese shows
+// '关闭'. Module-level tables are evaluated once, before a language can change: mark their strings N_('…') (a no-op that
 // tools/i18n.mjs extracts) and call t() where they are shown.
 //
 // Placeholders (the same in msgids and translations):
-//   {name}             the value of params.name (params may also be an array: {0}, {1} …)
-//   {n|one|other}      `one` when Number(params.n) === 1, else `other` — the only plural form (English needs no more)
-// A value that is an array renders as a list joined with the language's separator ('、' / ', '); a value `{ dn: '…' }`
-// (made by `dn()`) is a game-data name, translated through the name resolver the client installs (setNameResolver).
-// A placeholder whose value is missing stays verbatim, so official templates such as '{0}博士…' survive untouched.
+//   {name}                  the value of params.name (params may also be an array: {0}, {1} …)
+//   {n|one|other}           two plain forms: `one` when the language's plural category of Number(params.n) is 'one'
+//                           (Intl.PluralRules; in English exactly when n is 1), else `other`
+//   {n|one:…|few:…|other:…} named forms for languages with more categories (zero, one, two, few, many, other; `other`
+//                           is required): the form of the category, else `other`
+// A value that is an array renders as a list joined with the language's separator (the translation of 'list::、', '、' in
+// Chinese, ', ' without one); a value `{ dn: '…' }` (made by `dn()`) is a game-data name, translated through the name
+// resolver the client installs (setNameResolver). A placeholder whose value is missing stays verbatim, so official
+// templates such as '{0}博士…' survive untouched.
 //
 // Server → client messages (m.toast / m.ticker): the server builds `msg(msgid, params)` and sends
 // `wireMessage(m)` = { text: <the Chinese rendering>, msgid, params } — `text` keeps older clients working, the client
 // renders `translateWire(frame)` in its own language. A plain string message is its own msgid.
 
-/** Supported languages; the first one is the default and the source language of every msgid. */
-export const LANGS = Object.freeze(['zh', 'en']);
-export const DEFAULT_LANG = 'zh';
+import { SOURCE_LANG, canonicalLang, primaryLang, packMeta, computeChain } from './i18nPacks.js';
 
-/** List separator per language (array params). */
-const LIST_SEP = Object.freeze({ zh: '、', en: ', ' });
+/** The default language, the source language of every msgid. */
+export const DEFAULT_LANG = SOURCE_LANG;
 
+/** @typedef {import('./i18nPacks.js').LangMeta} LangMeta */
+
+/** The source language: built in, no pack, no catalog. */
+const SOURCE_META = Object.freeze({
+  code: SOURCE_LANG, name: '中文', englishName: 'Chinese (Simplified)', authors: [], credits: '', version: '', app: '', // i18n-ignore: its own name
+  base: null, fallback: [], complete: true, numberUnits: ['万', '亿'], builtin: true, // i18n-ignore: the Chinese number units
+});
+
+/** @type {Map<string, LangMeta>} code → metadata: the source, the packs of the index, languages given messages */
+const registry = new Map([[SOURCE_LANG, /** @type {LangMeta} */ (/** @type {unknown} */ (SOURCE_META))]]);
 /** @type {Map<string, Map<string, string>>} lang → msgid → translation */
 const catalogs = new Map();
 let current = DEFAULT_LANG;
 /** @type {Set<(lang: string, prev: string) => void>} */
 const listeners = new Set();
+/** @type {Set<() => void>} */
+const langsListeners = new Set();
 /** @type {((name: string, lang: string) => string) | null} */
 let nameResolver = null;
+/** @type {Map<string, string[]>} lang → its chain (cleared whenever the registry changes) */
+const chains = new Map();
+/** @type {Map<string, Map<string, { ok: boolean, problems: string[], extras: string[] }>>} lang → key → checkTranslation */
+const checked = new Map();
+/** @type {((info: { lang: string, key: string, problems: string[] }) => void) | null} */
+let warnHook = null;
+/** lang + key already reported (once each) */
+const warned = new Set();
+
+const report = (lang, key, problems) => {
+  if (!warnHook) return;
+  const id = `${lang}\u0000${key}`;
+  if (warned.has(id)) return;
+  warned.add(id);
+  try { warnHook({ lang, key, problems }); } catch { /* a broken hook never breaks a render */ }
+};
+
+const notifyLangs = () => {
+  chains.clear();
+  for (const fn of [...langsListeners]) {
+    try { fn(); } catch (err) { /** @type {any} */ (globalThis).console?.error('[i18n] listener failed', err); }
+  }
+};
 
 /**
- * A supported language code from loose input ('en-US' → 'en', 'zh_CN' → 'zh'), or null.
+ * Register languages (the client: the language entries of /packs/index.json, shared/packs.js langMetaOf). A known code
+ * is merged with what is there. Listeners of onLangsChange run once.
+ * @param {Partial<LangMeta>[]} entries each with a `code`
+ * @returns {number} languages registered
+ */
+export function registerLangs(entries) {
+  let n = 0;
+  for (const e of Array.isArray(entries) ? entries : []) {
+    const code = canonicalLang(e?.code);
+    if (!code || code === SOURCE_LANG) continue;
+    const prev = registry.get(code);
+    registry.set(code, /** @type {LangMeta} */ ({ ...(prev || packMeta(code, null)), ...e, code }));
+    n++;
+  }
+  if (n) notifyLangs();
+  return n;
+}
+
+/**
+ * The known languages, the source first, then by code: what the language menu lists.
+ * @returns {LangMeta[]}
+ */
+export function getLangs() {
+  const rest = [...registry.values()].filter((m) => m.code !== SOURCE_LANG).sort((a, b) => (a.code < b.code ? -1 : a.code > b.code ? 1 : 0));
+  return [registry.get(SOURCE_LANG), ...rest];
+}
+
+/** The metadata of a known language, or null. @param {string} [lang] */
+export const langInfo = (lang = current) => registry.get(String(lang)) || null;
+
+/**
+ * Subscribe to changes of the known languages (the index arrived, a pack loaded).
+ * @param {() => void} fn
+ * @returns {() => void} unsubscribe
+ */
+export function onLangsChange(fn) {
+  langsListeners.add(fn);
+  return () => { langsListeners.delete(fn); };
+}
+
+/**
+ * The languages a lookup tries for `lang`, best first; the Chinese msgid comes after them (i18nPacks.js computeChain).
+ * Empty for the source language.
+ * @param {string} [lang]
+ * @returns {string[]}
+ */
+export function langChain(lang = current) {
+  const code = String(lang);
+  if (code === SOURCE_LANG) return [];
+  let c = chains.get(code);
+  if (!c) {
+    c = computeChain(code, (x) => registry.get(x), (x) => registry.has(x) || catalogs.has(x));
+    chains.set(code, c);
+  }
+  return c;
+}
+
+/**
+ * A known language code from loose input ('en-US' → 'en' when 'en' is known, 'zh_CN' → 'zh', 'PT-br' → 'pt-BR'), or
+ * null: the exact code, then the same code in any case, then its language part.
  * @param {unknown} v
  * @returns {string|null}
  */
 export function normalizeLang(v) {
-  const s = typeof v === 'string' ? v.trim().toLowerCase() : '';
-  if (!s) return null;
-  for (const l of LANGS) if (s === l || s.startsWith(`${l}-`) || s.startsWith(`${l}_`)) return l;
-  return null;
+  const code = canonicalLang(v);
+  if (!code) return null;
+  if (registry.has(code)) return code;
+  const lower = code.toLowerCase();
+  for (const c of registry.keys()) if (c.toLowerCase() === lower) return c;
+  const p = primaryLang(code);
+  return registry.has(p) ? p : null;
 }
 
 /** The current language code. */
@@ -79,23 +185,30 @@ export function onLangChange(fn) {
 }
 
 /**
- * Add translations for a language (merged into what is there). Keys starting with '_' are metadata and skipped; only
- * non-empty string values count.
+ * Add translations for a language (merged into what is there; the language is registered — with the pack's `_meta`
+ * when the dict carries one). Keys starting with '_' are metadata and skipped; only non-empty string values count.
  * @param {string} lang
- * @param {Record<string, unknown> | null | undefined} dict msgid → translation
+ * @param {Record<string, unknown> | null | undefined} dict msgid → translation (a whole pack file)
  * @returns {number} entries added
  */
 export function addMessages(lang, dict) {
-  const l = normalizeLang(lang);
+  const l = canonicalLang(lang);
   if (!l || !dict || typeof dict !== 'object') return 0;
   let cat = catalogs.get(l);
   if (!cat) { cat = new Map(); catalogs.set(l, cat); }
+  checked.delete(l);
   let n = 0;
   for (const [k, v] of Object.entries(dict)) {
     if (k.startsWith('_') || typeof v !== 'string' || !v) continue;
     cat.set(k, v);
     n++;
   }
+  if (l !== SOURCE_LANG && !registry.get(l)?.packId && (dict._meta || !registry.has(l))) {
+    // a language the pack index did not list (no index, tests): what its own `_meta` says; an index entry (made from
+    // the pack's manifest by the server) stays as it is
+    registry.set(l, packMeta(l, dict));
+    notifyLangs();
+  } else chains.clear();
   return n;
 }
 
@@ -106,14 +219,15 @@ export function addMessages(lang, dict) {
  * @returns {number}
  */
 export function setMessages(lang, dict) {
-  const l = normalizeLang(lang);
+  const l = canonicalLang(lang);
   if (!l) return 0;
   catalogs.delete(l);
+  checked.delete(l);
   return addMessages(l, dict);
 }
 
 /**
- * Whether a msgid has a translation in a language (the default language always "has" its own msgids).
+ * Whether a msgid has a translation in a language's own pack (the default language always "has" its own msgids).
  * @param {string} msgid
  * @param {string} [lang]
  */
@@ -122,6 +236,13 @@ export function hasMessage(msgid, lang = current) {
   return l === DEFAULT_LANG || !!catalogs.get(l)?.has(String(msgid));
 }
 
+/**
+ * Install the reporter of skipped translations (a dropped or unknown placeholder, a broken plural form): called once per
+ * language and key. The client installs console.warn in development (ui/lang.js); null turns it off.
+ * @param {((info: { lang: string, key: string, problems: string[] }) => void) | null} fn
+ */
+export function setI18nWarn(fn) { warnHook = typeof fn === 'function' ? fn : null; warned.clear(); }
+
 /**
  * Install the translator of game-data names (`{ dn: '…' }` params and `tName`). The client maps a Chinese name to the
  * current language through the localized game data; without a resolver names stay Chinese.
@@ -150,6 +271,19 @@ export const dn = (name) => ({ dn: name == null ? '' : String(name) });
 
 const own = (o, k) => o != null && typeof o === 'object' && Object.prototype.hasOwnProperty.call(o, k);
 
+/**
+ * The list separator of a language: its translation of 'list::、' through its chain; '、' in Chinese; ', ' without one.
+ * @param {string} lang
+ */
+function listSep(lang) {
+  if (lang === DEFAULT_LANG) return '、';
+  for (const l of langChain(lang)) {
+    const s = catalogs.get(l)?.get('list::、');
+    if (s) return s;
+  }
+  return ', ';
+}
+
 /**
  * Render one param value.
  * @param {unknown} v
@@ -159,7 +293,7 @@ const own = (o, k) => o != null && typeof o === 'object' && Object.prototype.has
  */
 function renderValue(v, lang, resolve) {
   if (v == null) return '';
-  if (Array.isArray(v)) return v.map((x) => renderValue(x, lang, resolve)).filter((s) => s !== '').join(LIST_SEP[lang] || LIST_SEP.en);
+  if (Array.isArray(v)) return v.map((x) => renderValue(x, lang, resolve)).filter((s) => s !== '').join(listSep(lang));
   if (typeof v === 'object') {
     if (own(v, 'dn')) {
       const name = String(/** @type {any} */ (v).dn ?? '');
@@ -171,10 +305,72 @@ function renderValue(v, lang, resolve) {
   return String(v);
 }
 
-const PLACEHOLDER = /\{([A-Za-z0-9_$]+)(?:\|([^{}|]*)\|([^{}|]*))?\}/g;
+/** `{name}` or a plural `{name|form|form…}` (two or more forms); group 2 is the forms with their leading '|'. */
+const PLACEHOLDER = /\{([A-Za-z0-9_$]+)((?:\|[^{}|]*){2,})?\}/g;
+/** The CLDR plural categories (Intl.PluralRules). */
+export const PLURAL_CATEGORIES = Object.freeze(['zero', 'one', 'two', 'few', 'many', 'other']);
+const NAMED_FORM = /^(zero|one|two|few|many|other):/;
 
 /**
- * Fill a template's placeholders (pure: no global language state).
+ * The forms of a plural placeholder ('|second|seconds' or '|one:день|few:дня|many:дней|other:дня') by category, or null
+ * when malformed: two plain forms are one / other; named forms need `other` and no repeats; plain and named are not
+ * mixed, and three or more plain forms are refused (the order of a language's categories is not stable across engines).
+ * @param {string} forms the forms with their leading '|'
+ * @returns {Record<string, string> | null}
+ */
+export function parsePluralForms(forms) {
+  const parts = String(forms).slice(1).split('|');
+  const named = parts.map((p) => NAMED_FORM.exec(p));
+  if (named.every(Boolean)) {
+    /** @type {Record<string, string>} */
+    const out = {};
+    for (let i = 0; i < parts.length; i++) {
+      const cat = named[i][1];
+      if (own(out, cat)) return null;
+      out[cat] = parts[i].slice(named[i][0].length);
+    }
+    return own(out, 'other') ? out : null;
+  }
+  if (parts.length === 2 && !named.some(Boolean)) return { one: parts[0], other: parts[1] };
+  return null;
+}
+
+/** @type {Map<string, any>} lang → Intl.PluralRules (null: the runtime does not know the language) */
+const pluralRules = new Map();
+
+/**
+ * The plural category of a number in a language (Intl.PluralRules; a language the runtime does not know counts one
+ * for exactly 1, other for the rest — the English rule).
+ * @param {string} lang
+ * @param {number} n
+ * @returns {string}
+ */
+export function pluralCategory(lang, n) {
+  if (!Number.isFinite(n)) return 'other';
+  let pr = pluralRules.get(lang);
+  if (pr === undefined) {
+    pr = null;
+    try {
+      const PR = /** @type {any} */ (globalThis).Intl?.PluralRules;
+      const tag = PR && [lang, primaryLang(lang)].find((x) => PR.supportedLocalesOf([x]).length);
+      if (tag) pr = new PR(tag);
+    } catch { /* no Intl data for it */ }
+    pluralRules.set(lang, pr);
+  }
+  if (pr) return pr.select(n);
+  return n === 1 ? 'one' : 'other';
+}
+
+/** The form of a plural placeholder for a value (null when the forms are malformed). */
+function pluralForm(forms, v, lang) {
+  const f = parsePluralForms(forms);
+  if (!f) return null;
+  const cat = pluralCategory(lang, Number(v));
+  return own(f, cat) ? f[cat] : f.other;
+}
+
+/**
+ * Fill a template's placeholders (pure: no global language state but the list separator and plural rules of `lang`).
  * @param {unknown} template
  * @param {Record<string, unknown> | unknown[] | null | undefined} [params]
  * @param {{ lang?: string, resolveName?: ((name: string, lang: string) => string) | null }} [opts]
@@ -185,14 +381,79 @@ export function format(template, params, opts = {}) {
   if (!params || typeof params !== 'object' || !s.includes('{')) return s;
   const lang = normalizeLang(opts.lang) || DEFAULT_LANG;
   const resolve = opts.resolveName || null;
-  return s.replace(PLACEHOLDER, (m, key, one, other) => {
+  return s.replace(PLACEHOLDER, (m, key, forms) => {
     if (!own(params, key)) return m;
     const v = /** @type {any} */ (params)[key];
-    if (one !== undefined) return Number(v) === 1 ? one : other;
+    if (forms !== undefined) return pluralForm(forms, v, lang) ?? m;
     return renderValue(v, lang, resolve);
   });
 }
 
+/**
+ * Whether a translation fits its msgid: it keeps every plain placeholder of the msgid as a plain placeholder, and its
+ * plural forms parse. `extras`: the placeholder names it uses that the msgid lacks — allowed when the caller passes them
+ * (a plural selector beside a markup value: '{n} {count|unit|units}'), which t() checks per call and
+ * `node tools/i18n.mjs check` against the call sites.
+ * @param {string} msgid the msgid (for a tc() key, the part after '::')
+ * @param {string} text the translation
+ * @returns {{ ok: boolean, problems: string[], extras: string[] }}
+ */
+export function checkTranslation(msgid, text) {
+  const names = new Set();
+  const plain = new Set();
+  for (const m of String(msgid).matchAll(PLACEHOLDER)) {
+    names.add(m[1]);
+    if (m[2] === undefined) plain.add(m[1]);
+  }
+  const kept = new Set();
+  const extras = [];
+  const problems = [];
+  for (const m of String(text).matchAll(PLACEHOLDER)) {
+    if (m[2] === undefined) kept.add(m[1]);
+    else if (!parsePluralForms(m[2])) problems.push(`malformed plural ${m[0]}`);
+    if (!names.has(m[1]) && !extras.includes(m[1])) extras.push(m[1]);
+  }
+  for (const k of plain) if (!kept.has(k)) problems.push(`drops {${k}}`);
+  return { ok: problems.length === 0, problems, extras };
+}
+
+/** The checkTranslation of a catalog entry (cached per language). */
+function checkedEntry(lang, key, text) {
+  let m = checked.get(lang);
+  if (!m) { m = new Map(); checked.set(lang, m); }
+  let c = m.get(key);
+  if (!c) {
+    const sep = key.indexOf('::');
+    c = checkTranslation(sep >= 0 ? key.slice(sep + 2) : key, text);
+    m.set(key, c);
+  }
+  return c;
+}
+
+/**
+ * The translation of the first key (in order) that a language of the current chain has and that fits: in each
+ * language the keys in order (a tc() context key, then its plain msgid), then the next language. Null: show the msgid.
+ * @param {string[]} keys
+ * @param {Record<string, unknown> | unknown[] | null | undefined} params
+ * @returns {string|null}
+ */
+function lookup(keys, params) {
+  for (const l of langChain(current)) {
+    const cat = catalogs.get(l);
+    if (!cat) continue;
+    for (const key of keys) {
+      const tr = cat.get(key);
+      if (!tr) continue;
+      const c = checkedEntry(l, key, tr);
+      if (!c.ok) { report(l, key, c.problems); continue; }
+      const unknown = c.extras.filter((k) => !own(params, k));
+      if (unknown.length) { report(l, key, unknown.map((k) => `uses {${k}}, which the caller does not pass`)); continue; }
+      return tr;
+    }
+  }
+  return null;
+}
+
 /**
  * Translate a message into the current language (gettext style: the msgid is the Chinese text).
  * @param {unknown} msgid
@@ -201,11 +462,7 @@ export function format(template, params, opts = {}) {
  */
 export function t(msgid, params) {
   const id = msgid == null ? '' : String(msgid);
-  let text = id;
-  if (current !== DEFAULT_LANG) {
-    const tr = catalogs.get(current)?.get(id);
-    if (tr) text = tr;
-  }
+  const text = current === DEFAULT_LANG ? id : lookup([id], params) ?? id;
   return params ? format(text, params, { lang: current, resolveName: nameResolver }) : text;
 }
 
@@ -218,11 +475,8 @@ export function t(msgid, params) {
  */
 export function tc(context, msgid, params) {
   const id = msgid == null ? '' : String(msgid);
-  if (current !== DEFAULT_LANG) {
-    const tr = catalogs.get(current)?.get(`${context}::${id}`);
-    if (tr) return params ? format(tr, params, { lang: current, resolveName: nameResolver }) : tr;
-  }
-  return t(id, params);
+  const text = current === DEFAULT_LANG ? id : lookup([`${context}::${id}`, id], params) ?? id;
+  return params ? format(text, params, { lang: current, resolveName: nameResolver }) : text;
 }
 
 /** A tParts() param kept as it is: an object that is not a `{ dn }` name (a vnode), or an array holding one. */
@@ -231,7 +485,7 @@ const isMarkup = (v) => !!v && typeof v === 'object' && !own(v, 'dn') && (!Array
 /**
  * A message as pieces for a renderer, for a sentence with markup inside (「第 <b>14</b> 回合」): the translation split at
  * its placeholders; a param that is markup (a vnode, or an array holding vnodes) is kept as it is, the others render as
- * in t() ({ dn } names, lists, {n|one|other}). Adjacent text joins, empty text is dropped, so the Chinese pieces are the
+ * in t() ({ dn } names, lists, plural forms). Adjacent text joins, empty text is dropped, so the Chinese pieces are the
  * text around the markup exactly as written before:
  *   html`<span>${tParts('第 {r} 回合 · 最终攻势', { r: html`<b class="num">${n}</b>` })}</span>`
  * @param {unknown} msgid
@@ -240,11 +494,7 @@ const isMarkup = (v) => !!v && typeof v === 'object' && !own(v, 'dn') && (!Array
  */
 export function tParts(msgid, params) {
   const id = msgid == null ? '' : String(msgid);
-  let text = id;
-  if (current !== DEFAULT_LANG) {
-    const tr = catalogs.get(current)?.get(id);
-    if (tr) text = tr;
-  }
+  const text = current === DEFAULT_LANG ? id : lookup([id], params) ?? id;
   /** @type {unknown[]} */
   const out = [];
   const push = (s) => {
@@ -255,11 +505,11 @@ export function tParts(msgid, params) {
   let last = 0;
   if (params && typeof params === 'object') {
     for (const m of text.matchAll(PLACEHOLDER)) {
-      const [whole, key, one, other] = m;
+      const [whole, key, forms] = m;
       if (!own(params, key)) continue;
       push(text.slice(last, m.index));
       const v = /** @type {any} */ (params)[key];
-      if (one !== undefined) push(Number(v) === 1 ? one : other);
+      if (forms !== undefined) push(pluralForm(forms, v, current) ?? whole);
       else if (isMarkup(v)) out.push(v);
       else push(renderValue(v, current, nameResolver));
       last = m.index + whole.length;

shared/i18nData.js

@@ -10,6 +10,11 @@
 // the 2-character hash (textHash2) of each Chinese string a leaf replaces, in walk order (object keys in their order,
 // array entries by index, `_h` itself skipped). A record that is a plain string (config `seasonName`) has { _s, _h }. A leaf whose Chinese source no longer hashes the same — data/*.json was
 // rebuilt after the overlay — is not applied: the player sees the current Chinese text rather than a stale translation.
+//
+// Language packs (shared/i18nPacks.js): a language's game texts may come from several overlays — the pack's own, its
+// base's, a fallback's (English) — applied per text: `applyFileOverlay(json, overlay, onto)` checks each leaf against
+// the Chinese `json` and writes it over `onto` (what the overlays of lower priority made), so applying them from the
+// last of the chain to the first leaves, for every text, the best overlay that has it (public/js/data.js).
 
 export const OVERLAY_VERSION = 1;
 
@@ -106,77 +111,83 @@ export function buildRecordOverlay(leaves) {
 
 /**
  * Apply a record overlay to a Chinese record: a copy that shares every untouched branch (copy-on-write), with each leaf
- * whose Chinese source still matches its hash replaced. The input record is never modified.
+ * whose Chinese source still matches its hash replaced. The input record is never modified. `onto` (default: the
+ * record itself) is what the replaced leaves are written over — the record as overlays of lower priority left it — while
+ * the hashes are always checked against the Chinese `base`.
  * @param {any} base
  * @param {any} ov
+ * @param {any} [onto]
  * @returns {{ value: any, applied: number, stale: number }}
  */
-export function applyRecordOverlay(base, ov) {
+export function applyRecordOverlay(base, ov, onto = base) {
   if (isObj(ov) && typeof ov._s === 'string') {
     const ok = typeof base === 'string' && textHash2(base) === ov._h;
-    return { value: ok ? ov._s : base, applied: ok ? 1 : 0, stale: ok ? 0 : 1 };
+    return { value: ok ? ov._s : onto, applied: ok ? 1 : 0, stale: ok ? 0 : 1 };
   }
-  if (!isObj(ov) || base == null || typeof base !== 'object') return { value: base, applied: 0, stale: 0 };
+  if (!isObj(ov) || base == null || typeof base !== 'object') return { value: onto, applied: 0, stale: 0 };
   const hashes = typeof ov._h === 'string' ? ov._h : '';
   let i = 0;
   let applied = 0;
   let stale = 0;
-  const rec = (b, o, depth) => {
+  // b: the Chinese at this path (hash check); t: the target at this path (same shape: overlays replace strings only)
+  const rec = (b, t, o, depth) => {
     if (typeof o === 'string') {
       const want = hashes.slice(i * 2, i * 2 + 2);
       i++;
       if (typeof b === 'string' && want && textHash2(b) === want) { applied++; return o; }
       stale++;
-      return b;
+      return t;
     }
     if (Array.isArray(o)) {
-      if (!Array.isArray(b)) { walkOverlay(o, () => { i++; stale++; }); return b; }
-      let out = b;
+      if (!Array.isArray(b) || !Array.isArray(t)) { walkOverlay(o, () => { i++; stale++; }); return t; }
+      let out = t;
       for (let k = 0; k < o.length; k++) {
         if (o[k] == null) continue;
-        const nv = rec(b[k], o[k], depth + 1);
-        if (nv !== b[k]) { if (out === b) out = b.slice(); out[k] = nv; }
+        const nv = rec(b[k], t[k], o[k], depth + 1);
+        if (nv !== t[k]) { if (out === t) out = t.slice(); out[k] = nv; }
       }
       return out;
     }
     if (isObj(o)) {
-      if (b == null || typeof b !== 'object') { walkOverlay(o, () => { i++; stale++; }); return b; }
+      if (b == null || typeof b !== 'object' || t == null || typeof t !== 'object') { walkOverlay(o, () => { i++; stale++; }); return t; }
       // an array record's overlay is keyed by index (see buildRecordOverlay)
       const arr = Array.isArray(b);
-      let out = b;
+      let out = t;
       for (const k of Object.keys(o)) {
         if (depth === 0 && k === '_h') continue;
         const at = arr ? (/^\d+$/.test(k) ? Number(k) : -1) : k;
         const cur = at === -1 ? undefined : b[at];
-        const nv = rec(cur, o[k], depth + 1);
-        if (at !== -1 && nv !== cur) { if (out === b) out = arr ? b.slice() : { ...b }; out[at] = nv; }
+        const tcur = at === -1 ? undefined : t[at];
+        const nv = rec(cur, tcur, o[k], depth + 1);
+        if (at !== -1 && nv !== tcur) { if (out === t) out = Array.isArray(t) ? t.slice() : { ...t }; out[at] = nv; }
       }
       return out;
     }
-    return b;
+    return t;
   };
-  const value = rec(base, ov, 0);
+  const value = rec(base, onto, ov, 0);
   return { value, applied, stale };
 }
 
 /**
  * Apply a file's overlay to a whole data file (the `{ id: record }` maps of data/*.json; other shapes are returned
- * unchanged). Untouched records are shared with the input.
+ * unchanged). Untouched records are shared with the input. `onto`: see applyRecordOverlay (a whole file).
  * @param {any} json the Chinese file
  * @param {Record<string, any> | null | undefined} fileOverlay `files[<name>]` of the overlay
+ * @param {any} [onto] the file as overlays of lower priority left it (default: the Chinese file)
  * @returns {{ value: any, applied: number, stale: number }}
  */
-export function applyFileOverlay(json, fileOverlay) {
-  if (!isObj(json) || !isObj(fileOverlay)) return { value: json, applied: 0, stale: 0 };
-  let out = json;
+export function applyFileOverlay(json, fileOverlay, onto = json) {
+  if (!isObj(json) || !isObj(fileOverlay) || !isObj(onto)) return { value: onto, applied: 0, stale: 0 };
+  let out = onto;
   let applied = 0;
   let stale = 0;
   for (const [id, ov] of Object.entries(fileOverlay)) {
     if (!Object.prototype.hasOwnProperty.call(json, id)) continue;
-    const r = applyRecordOverlay(json[id], ov);
+    const r = applyRecordOverlay(json[id], ov, onto[id]);
     applied += r.applied;
     stale += r.stale;
-    if (r.value !== json[id]) { if (out === json) out = { ...json }; out[id] = r.value; }
+    if (r.value !== onto[id]) { if (out === onto) out = { ...onto }; out[id] = r.value; }
   }
   return { value: out, applied, stale };
 }

shared/i18nPacks.js

@@ -0,0 +1,171 @@
+// shared/i18nPacks.js — the language pack type ("lang") of the content packs (shared/packs.js, docs/PACKS.md;
+// translating: docs/I18N.md "Adding a language"): language codes, the language fields of a pack manifest, the fallback
+// chain t() walks, and the script family the title screen lays out by. No Node builtin: the browser, the server and the
+// tools share it.
+//
+// A language pack is files in the language folders, nothing else (the owner's decision of 2026-10-07):
+//
+//   public/i18n/<code>.json   the UI strings: { "_meta": { …manifest… }, "<Chinese msgid>": "<translation>", … }
+//   data/i18n/<code>.json     optional: the game texts (the overlay of shared/i18nData.js, tools/build-i18n.mjs)
+//
+// — or the same two files in a pack folder, packs/<id>/ with a pack.json manifest (shared/packs.js), the layout every
+// pack type shares. <code> is a BCP 47 tag in its usual case: a 2–3 letter language, then an optional script (4
+// letters), region (2 letters or 3 digits) and variants — en, ja, ko, zh-TW, pt-BR, sr-Latn. Chinese ('zh') is the
+// built-in source language (the msgids themselves) and never a pack. English is a pack like any other; it is only the
+// complete one.
+//
+// The language fields of a manifest (`_meta` of a single-file pack, pack.json of a folder pack; all optional):
+//   lang          the language code (a single file: its file name decides; a different value is reported)
+//   base          a pack whose strings fill the gaps first (default: the language of a regional code when there is a
+//                 pack for it — pt-BR → pt; never the Chinese source, so zh-TW has none); null switches it off
+//   fallback      packs tried after the base, before the Chinese msgid (["en"]: untranslated strings show English)
+//   complete      true: `node tools/i18n.mjs check` requires every msgid (English declares it)
+//   numberUnits   two units for numbers counted in 10⁴ / 10⁸ steps (Chinese 万 / 亿; e.g. a Japanese pack ["万", "億"]);
+//                 without them large numbers use K / M / B (ui/gameLogic/format.js fmtNum)
+// The common fields (id, version, app, name, englishName, authors, credits, license) are shared/packs.js's.
+
+/** The source language: msgids are its text, it has no pack. */
+export const SOURCE_LANG = 'zh';
+
+/**
+ * A language code in its canonical case ('EN' → 'en', 'pt_br' → 'pt-BR', 'zh-hant-tw' → 'zh-Hant-TW'), or null when
+ * the input is not one.
+ * @param {unknown} v
+ * @returns {string|null}
+ */
+export function canonicalLang(v) {
+  if (typeof v !== 'string') return null;
+  const parts = v.trim().replace(/_/g, '-').split('-');
+  if (!/^[A-Za-z]{2,3}$/.test(parts[0] || '')) return null;
+  const out = [parts[0].toLowerCase()];
+  for (let i = 1; i < parts.length; i++) {
+    const p = parts[i];
+    if (!/^[A-Za-z0-9]{2,8}$/.test(p)) return null;
+    if (i === 1 && /^[A-Za-z]{4}$/.test(p)) out.push(p[0].toUpperCase() + p.slice(1).toLowerCase()); // script
+    else if (/^[A-Za-z]{2}$/.test(p) || /^\d{3}$/.test(p)) out.push(p.toUpperCase()); // region
+    else out.push(p.toLowerCase()); // variant
+  }
+  return out.join('-');
+}
+
+/** Whether a string is a language code in its canonical case (a pack's file name must be one). @param {unknown} v */
+export const isLangCode = (v) => typeof v === 'string' && canonicalLang(v) === v;
+
+/** The language part of a code ('pt-BR' → 'pt'). @param {string} code */
+export const primaryLang = (code) => String(code).split('-')[0];
+
+/**
+ * A language's name through Intl.DisplayNames (`code` in the language `inLang`), or the code when unknown.
+ * @param {string} code
+ * @param {string} [inLang] default: the language itself (日本語 for ja)
+ */
+export function languageName(code, inLang = code) {
+  try {
+    const DN = /** @type {any} */ (globalThis).Intl?.DisplayNames;
+    if (DN) {
+      const n = new DN([inLang], { type: 'language', fallback: 'none' }).of(code);
+      if (typeof n === 'string' && n && n !== code) return n;
+    }
+  } catch { /* an unknown code or no ICU data */ }
+  return code;
+}
+
+const str = (v, max = 200) => (typeof v === 'string' && v.trim() ? v.trim().slice(0, max) : '');
+
+/** Canonical language codes of a string or a list, the source and repeats left out. @param {unknown} v */
+export function langList(v) {
+  const list = Array.isArray(v) ? v : typeof v === 'string' ? [v] : [];
+  const out = [];
+  for (const x of list) {
+    const c = canonicalLang(x);
+    if (c && c !== SOURCE_LANG && !out.includes(c)) out.push(c);
+  }
+  return out;
+}
+
+/**
+ * @typedef {{ lang: string, base: string|null|undefined, fallback: string[], complete: boolean, numberUnits: string[]|null }} LangFields
+ */
+
+/**
+ * The language fields of a manifest (see the header), normalized. `base` stays undefined unless the manifest sets it
+ * (a code or null): computeChain() then takes the language of a regional code.
+ * @param {string} code the pack's language
+ * @param {any} m the manifest (`_meta` / pack.json), may be empty
+ * @returns {LangFields}
+ */
+export function langFields(code, m) {
+  const o = m && typeof m === 'object' ? m : {};
+  let base;
+  if (o.base === null) base = null;
+  else if (typeof o.base === 'string') base = canonicalLang(o.base);
+  if (base === SOURCE_LANG || base === code) base = null;
+  const units = Array.isArray(o.numberUnits) && o.numberUnits.length === 2 && o.numberUnits.every((u) => str(u, 8)) ? o.numberUnits.map((u) => str(u, 8)) : null;
+  return { lang: code, base, fallback: langList(o.fallback).filter((c) => c !== code), complete: o.complete === true, numberUnits: units };
+}
+
+/**
+ * @typedef {LangFields & { code: string, name: string, englishName: string, authors: string[], credits?: string,
+ *   version?: string, app?: string, compatible?: boolean, data?: boolean, strings?: number, ui?: string,
+ *   dataUrl?: string, packId?: string, builtin?: boolean }} LangMeta
+ */
+
+/**
+ * What the client knows of a language (shared/i18n.js registry) from a pack's UI file alone — its `_meta`, every field
+ * optional: the names default to Intl.DisplayNames, no authors, no fallback.
+ * @param {string} code
+ * @param {any} json the parsed UI file (or null)
+ * @returns {LangMeta}
+ */
+export function packMeta(code, json) {
+  const m = json && typeof json === 'object' && json._meta && typeof json._meta === 'object' ? json._meta : {};
+  const authors = (Array.isArray(m.authors) ? m.authors : typeof m.authors === 'string' ? [m.authors] : []).map((a) => str(a)).filter(Boolean).slice(0, 32);
+  return {
+    ...langFields(code, m),
+    code,
+    name: str(m.name, 60) || languageName(code),
+    englishName: str(m.englishName, 60) || languageName(code, 'en'),
+    authors,
+  };
+}
+
+/** How many translations a UI file holds (non-empty string values, metadata keys left out). @param {any} json */
+export function countStrings(json) {
+  let n = 0;
+  if (json && typeof json === 'object') for (const [k, v] of Object.entries(json)) if (!k.startsWith('_') && typeof v === 'string' && v) n++;
+  return n;
+}
+
+/**
+ * The languages t() tries for `code`, best first (the Chinese msgid comes after all of them): the pack itself, its base
+ * — `base`, else the language of a regional code (pt-BR → pt) — with the base's own chain, then each `fallback` with its
+ * chain. Only languages `known` accepts; never the source; no repeats (cycles end).
+ * @param {string} code
+ * @param {(code: string) => Partial<LangFields> | null | undefined} metaOf
+ * @param {(code: string) => boolean} known
+ * @returns {string[]}
+ */
+export function computeChain(code, metaOf, known) {
+  const out = [];
+  const visit = (c, depth) => {
+    if (!c || c === SOURCE_LANG || out.includes(c) || depth > 8 || !known(c)) return;
+    out.push(c);
+    const m = metaOf(c) || {};
+    const base = m.base !== undefined ? m.base : (c.includes('-') ? primaryLang(c) : null);
+    if (base) visit(base, depth + 1);
+    for (const f of m.fallback || []) visit(f, depth + 1);
+  };
+  visit(code, 0);
+  return out;
+}
+
+/** Characters of the wide scripts: CJK ideographs and punctuation, kana, Hangul, full-width forms. */
+const WIDE_SCRIPT = /[ᄀ-ᇿ⺀-鿿가-힯豈-﫿＀-￯]/;
+
+/**
+ * The script family of a text: 'cjk' (ideographs, kana, Hangul) or 'alphabetic' (Latin, Cyrillic, Greek …). The title
+ * screen and the start button lay out by it (screens/title.js, ui/lang.js applyDocument), so a pack needs no flag.
+ * @param {unknown} text
+ * @returns {'cjk' | 'alphabetic'}
+ */
+export const scriptOf = (text) => (WIDE_SCRIPT.test(String(text ?? '')) ? 'cjk' : 'alphabetic');

shared/packs.js

@@ -0,0 +1,285 @@
+// shared/packs.js — content packs (docs/PACKS.md): one manifest format, one registry, one index for every kind of
+// add-on content (the owner's decision of 2026-10-07: the language pack is the first type of a general mechanism). A
+// pack is a manifest plus its files. Type "lang" (language packs, shared/i18nPacks.js) is the first and, in 0.2.0, the
+// only type that loads; "assets" (art / audio / font replacements, client side) and "data" (data patches the server
+// applies at start) are planned: their manifests already parse, a server lists them as not supported yet, and each will
+// plug in as its entry in PACK_TYPES plus its loader (docs/PACKS.md "Adding a pack type"). No Node builtin: the
+// browser, the server (server/packs.js) and the tools share it.
+//
+// Two layouts, read by the same registry (server/packs.js) and listed alike:
+//   packs/<id>/pack.json + its files   a folder pack — any type; the manifest names its files (`files`)
+//   public/i18n/<code>.json            a single-file language pack — its manifest is the file's `_meta` block
+//                                      (+ data/i18n/<code>.json, its game texts, when present)
+//
+// Manifest (pack.json, or `_meta` of a single-file language pack; every field but type / lang optional):
+//   id            a folder pack: its folder name; a single-file language pack: its language code (a manifest that names
+//                 another id is reported; the place decides). Letters, digits, '.', '_', '-', ≤ 64; unique across types
+//   type          "lang" (a single-file language pack may leave it out); planned: "assets", "data"
+//   version       the pack's own version (e.g. "1.2.0")
+//   app           the app versions it is made for, a range: ">=0.2.0", ">=0.2.0 <0.3.0", "0.2.x", "^0.2.0", "~0.2.1", "*"
+//                 (a pre-release such as 0.2.0-dev counts as its release). Outside it a language pack still loads —
+//                 what it lacks falls back — and is flagged `compatible: false`
+//   name          the pack's name in its own language (a language pack: the language's own name, shown in the menu)
+//   englishName   its English name
+//   authors       a string or a list;  credits  free text;  license  an SPDX id or free text
+//   files         a folder pack: role → path inside the folder (type "lang": { "ui": "ui.json", "data": "data.json" })
+//   …             the type's own fields (type "lang": lang, base, fallback, complete, numberUnits — shared/i18nPacks.js)
+//
+// Index (GET /packs/index.json — the server's live list; `node tools/packs.mjs index` writes packs/index.json for a
+// static host): { version: 1, app: "<the server's app version>", packs: [entry …] } — the packs of supported types,
+// sorted by type then id. An entry: id, type, name, englishName, version?, app?, compatible, authors?, credits?,
+// license?, files (role → URL on this server) and the type's fields; a language entry: lang, base?, fallback,
+// complete?, numberUnits?, strings (how many translations its UI file holds).
+
+import { APP_VERSION } from './constants.js';
+import { SOURCE_LANG, canonicalLang, isLangCode, languageName, langFields } from './i18nPacks.js';
+
+export const PACK_INDEX_VERSION = 1;
+/** A folder pack's manifest file. */
+export const PACK_MANIFEST = 'pack.json';
+/** The index file next to the pack folders (packs/index.json, URL /packs/index.json). */
+export const PACK_INDEX_FILE = 'index.json';
+/** Where pack folders are served (server/http/static.js): /packs/<id>/<file>. */
+export const PACKS_URL = '/packs/';
+
+/**
+ * @typedef {{ required?: boolean, ext: string[] }} FileRole a role of a folder pack's `files`
+ * @typedef {{ status: 'supported' | 'planned', summary: string, live?: boolean, files?: Record<string, FileRole> }} PackType
+ *   live: a server picks up its changes without a restart (lang: the client loads packs on demand; data will not —
+ *   the simulation's data is fixed at start). files: the roles a folder pack of the type names, with the extensions a
+ *   server may serve for them (nothing else in a pack folder is ever served)
+ */
+
+/** @type {Readonly<Record<string, PackType>>} the pack types (docs/PACKS.md "Adding a pack type") */
+export const PACK_TYPES = Object.freeze({
+  lang: Object.freeze({
+    status: 'supported', live: true, summary: 'a language: UI strings and, optionally, game texts',
+    files: Object.freeze({ ui: Object.freeze({ required: true, ext: ['.json'] }), data: Object.freeze({ ext: ['.json'] }) }),
+  }),
+  assets: Object.freeze({ status: 'planned', summary: 'art, audio and font replacements, client side' }),
+  data: Object.freeze({ status: 'planned', summary: 'data patches the server applies at start' }),
+});
+
+const isObj = (v) => !!v && typeof v === 'object' && !Array.isArray(v);
+const str = (v, max = 200) => (typeof v === 'string' && v.trim() ? v.trim().slice(0, max) : '');
+
+/** A pack id: letters, digits, '.', '_', '-' (not first), at most 64. @param {unknown} v */
+export const isPackId = (v) => typeof v === 'string' && /^[A-Za-z0-9][A-Za-z0-9._-]{0,63}$/.test(v);
+
+/**
+ * A path inside a pack folder ('ui.json', 'img/a.png'): relative, '/'-separated, no '.' / '..' / hidden / empty segment,
+ * plain characters only.
+ * @param {unknown} rel
+ */
+export const isPackPath = (rel) => typeof rel === 'string' && rel.length <= 200 && /^[A-Za-z0-9._ -]+(?:\/[A-Za-z0-9._ -]+)*$/.test(rel)
+  && rel.split('/').every((s) => s.trim() === s && s && !s.startsWith('.'));
+
+const extOf = (rel) => { const m = /\.[A-Za-z0-9]+$/.exec(rel); return m ? m[0].toLowerCase() : ''; };
+
+// ---- app version ranges -------------------------------------------------------------------------------------------
+
+/** [major, minor, patch] of a version ('0.2.0-dev' → [0, 2, 0]: a pre-release counts as its release), or null. */
+const versionCore = (v) => {
+  const m = /^v?(\d+)\.(\d+)\.(\d+)/.exec(String(v ?? '').trim());
+  return m ? [Number(m[1]), Number(m[2]), Number(m[3])] : null;
+};
+const cmp = (a, b) => a[0] - b[0] || a[1] - b[1] || a[2] - b[2];
+
+/** One comparator of a range as [op, version] bounds, or null when it is not one. */
+function bounds(c) {
+  if (c === '*' || c === 'x') return [];
+  const m = /^(\^|~|>=|<=|>|<|=)?v?(\d+)(?:\.(\d+|[xX*]))?(?:\.(\d+|[xX*]))?$/.exec(c);
+  if (!m) return null;
+  const [, op = '', a, b, d] = m;
+  const wild = (s) => s === undefined || /^[xX*]$/.test(s);
+  const M = Number(a);
+  const N = wild(b) ? 0 : Number(b);
+  const P = wild(d) ? 0 : Number(d);
+  const partial = wild(b) ? 1 : wild(d) ? 2 : 3; // how many parts were given
+  const lo = [M, N, P];
+  if (op === '^') return [['>=', lo], ['<', M > 0 ? [M + 1, 0, 0] : partial >= 2 ? [0, N + 1, 0] : [1, 0, 0]]];
+  if (op === '~') return [['>=', lo], ['<', partial >= 2 ? [M, N + 1, 0] : [M + 1, 0, 0]]];
+  if (op === '' || op === '=') {
+    if (partial === 3) return [['=', lo]];
+    return [['>=', lo], ['<', partial === 2 ? [M, N + 1, 0] : [M + 1, 0, 0]]];
+  }
+  return [[op, lo]];
+}
+
+/** Whether `range` is a version range this file understands (an empty one is). @param {unknown} range */
+export function isVersionRange(range) {
+  const r = String(range ?? '').trim();
+  if (!r) return true;
+  return r.split('||').every((alt) => alt.trim().split(/\s+/).every((c) => bounds(c) !== null));
+}
+
+/**
+ * Whether an app version is in a pack's `app` range (see the header). An empty range, an unreadable range or version
+ * counts as a match: the range is advice, the pack's loader decides what an incompatible pack means.
+ * @param {unknown} range
+ * @param {unknown} [version] default: this build's APP_VERSION
+ */
+export function appVersionMatches(range, version = APP_VERSION) {
+  const v = versionCore(version);
+  const r = String(range ?? '').trim();
+  if (!v || !r || !isVersionRange(r)) return true;
+  return r.split('||').some((alt) => alt.trim().split(/\s+/).every((c) => bounds(c).every(([op, b]) => {
+    const d = cmp(v, b);
+    return op === '>=' ? d >= 0 : op === '>' ? d > 0 : op === '<=' ? d <= 0 : op === '<' ? d < 0 : d === 0;
+  })));
+}
+
+// ---- manifests ----------------------------------------------------------------------------------------------------
+
+/**
+ * @typedef {{ id: string, type: string, name: string, englishName: string, version: string, app: string,
+ *   compatible: boolean, authors: string[], credits: string, license: string, files: Record<string, string>,
+ *   lang?: string, base?: string|null, fallback?: string[], complete?: boolean, numberUnits?: string[]|null }} PackManifest
+ */
+
+/**
+ * Read a manifest: the common fields, the type's fields, a folder pack's file roles. `problems` keep the pack from
+ * loading (manifest null); `warnings` do not.
+ * @param {unknown} raw pack.json, or the `_meta` of a single-file language pack (undefined: none)
+ * @param {{ id: string, type?: string, lang?: string, folder?: boolean, app?: string }} ctx
+ *   id: the folder name / the file's code; type: the type of the layout (a single-file language pack: 'lang');
+ *   lang: a single file's language code; folder: a folder pack (its `files` are read); app: the app version
+ * @returns {{ manifest: PackManifest | null, problems: string[], warnings: string[] }}
+ */
+export function normalizeManifest(raw, ctx) {
+  const problems = [];
+  const warnings = [];
+  if (raw !== undefined && !isObj(raw)) problems.push('the manifest is not a JSON object');
+  const m = isObj(raw) ? /** @type {any} */ (raw) : {};
+  const type = str(m.type, 20) || ctx.type || '';
+  const kind = Object.prototype.hasOwnProperty.call(PACK_TYPES, type) ? PACK_TYPES[type] : null;
+  if (!type) problems.push('no "type"');
+  else if (!kind) problems.push(`unknown type "${type}" (known: ${Object.keys(PACK_TYPES).join(', ')})`);
+  if (ctx.type && type !== ctx.type) problems.push(`"type": "${type}" where a ${ctx.type} pack belongs`);
+  const id = ctx.id;
+  if (!isPackId(id)) problems.push(`"${id}" is not a pack id (letters, digits, . _ -)`);
+  if (m.id !== undefined && m.id !== id) warnings.push(`"id": "${m.id}" is not "${id}" (the folder or file name decides)`);
+  const range = str(m.app, 60);
+  if (range && !isVersionRange(range)) warnings.push(`"app": "${range}" is not a version range (">=0.2.0", "0.2.x" …)`);
+  const app = ctx.app ?? APP_VERSION;
+  const compatible = appVersionMatches(range, app);
+  if (!compatible) warnings.push(`made for app ${range}; this is ${app}`);
+  const authors = (Array.isArray(m.authors) ? m.authors : typeof m.authors === 'string' ? [m.authors] : []).map((a) => str(a)).filter(Boolean).slice(0, 32);
+  /** @type {PackManifest} */
+  const out = {
+    id, type, name: str(m.name, 60), englishName: str(m.englishName, 60), version: str(m.version, 40), app: range, compatible,
+    authors, credits: str(m.credits, 500), license: str(m.license, 100), files: {},
+  };
+  if (type === 'lang') {
+    const code = ctx.lang || canonicalLang(m.lang);
+    if (ctx.lang && m.lang !== undefined && canonicalLang(m.lang) !== ctx.lang) warnings.push(`"lang": "${m.lang}" is not its file's code ${ctx.lang}`);
+    if (!code) problems.push(m.lang === undefined ? 'no "lang" (the language code)' : `"lang": "${m.lang}" is not a language code`);
+    else if (code === SOURCE_LANG) problems.push('"zh" is the source language (the msgids), never a pack');
+    else {
+      if (m.lang !== undefined && !isLangCode(m.lang) && !ctx.lang) warnings.push(`"lang": "${m.lang}" read as ${code}`);
+      Object.assign(out, langFields(code, m));
+      out.name ||= languageName(code);
+      out.englishName ||= languageName(code, 'en');
+    }
+  }
+  out.name ||= id;
+  out.englishName ||= out.name;
+  if (ctx.folder && kind && kind.files) {
+    const given = isObj(m.files) ? m.files : {};
+    for (const [role, spec] of Object.entries(kind.files)) {
+      const rel = given[role];
+      if (rel === undefined || rel === null || rel === '') { if (spec.required) problems.push(`no "files.${role}"`); continue; }
+      if (!isPackPath(rel)) problems.push(`"files.${role}": "${rel}" is not a path inside the pack folder`);
+      else if (!spec.ext.includes(extOf(rel))) problems.push(`"files.${role}" must be a ${spec.ext.join(' / ')} file`);
+      else out.files[role] = rel;
+    }
+    for (const role of Object.keys(given)) if (!kind.files[role]) warnings.push(`"files.${role}" is not a role of a ${type} pack (ignored)`);
+  }
+  return { manifest: problems.length ? null : out, problems, warnings };
+}
+
+// ---- the index ----------------------------------------------------------------------------------------------------
+
+/**
+ * The index entry of a pack (what a client may know): its manifest without the folder paths, `files` as URLs, plus the
+ * type's extras (a language pack: `strings`).
+ * @param {PackManifest} manifest
+ * @param {{ files: Record<string, string>, strings?: number }} extra files: role → URL
+ */
+export function packIndexEntry(manifest, extra) {
+  /** @type {Record<string, unknown>} */
+  const e = { id: manifest.id, type: manifest.type, name: manifest.name, englishName: manifest.englishName };
+  if (manifest.version) e.version = manifest.version;
+  if (manifest.app) e.app = manifest.app;
+  e.compatible = manifest.compatible;
+  if (manifest.authors.length) e.authors = manifest.authors;
+  if (manifest.credits) e.credits = manifest.credits;
+  if (manifest.license) e.license = manifest.license;
+  if (manifest.type === 'lang') {
+    e.lang = manifest.lang;
+    if (manifest.base !== undefined) e.base = manifest.base;
+    e.fallback = manifest.fallback || [];
+    if (manifest.complete) e.complete = true;
+    if (manifest.numberUnits) e.numberUnits = manifest.numberUnits;
+    e.strings = extra.strings ?? 0;
+  }
+  e.files = { ...extra.files };
+  return e;
+}
+
+/**
+ * The index body from the entries of supported packs (packIndexEntry), sorted by type then id.
+ * @param {Record<string, unknown>[]} entries
+ * @param {{ app?: string }} [opts]
+ */
+export function buildPackIndex(entries, { app = APP_VERSION } = {}) {
+  const key = (e) => `${e.type}\u0000${e.id}`;
+  return { version: PACK_INDEX_VERSION, app, packs: [...entries].sort((a, b) => (key(a) < key(b) ? -1 : key(a) > key(b) ? 1 : 0)) };
+}
+
+/** A URL a client may fetch from an index: a path on this server ('/…', never '//host' or a scheme). */
+const isLocalUrl = (u) => typeof u === 'string' && /^\/(?!\/)[^\s\\]*$/.test(u) && u.length <= 300;
+
+/**
+ * Read a received index (the client): the entries of supported types whose fields check out; anything else is dropped.
+ * Optionally only one type.
+ * @param {unknown} json
+ * @param {string} [type]
+ * @returns {Record<string, any>[]}
+ */
+export function readPackIndex(json, type) {
+  const list = isObj(json) && Array.isArray(/** @type {any} */ (json).packs) ? /** @type {any} */ (json).packs : [];
+  const out = [];
+  for (const e of list) {
+    if (!isObj(e) || !isPackId(e.id) || PACK_TYPES[e.type]?.status !== 'supported' || (type && e.type !== type)) continue;
+    const files = {};
+    for (const [role, url] of Object.entries(isObj(e.files) ? e.files : {})) if (isLocalUrl(url)) files[role] = url;
+    if (e.type === 'lang' && (!isLangCode(e.lang) || e.lang === SOURCE_LANG || !files.ui)) continue;
+    out.push({ ...e, files });
+  }
+  return out;
+}
+
+/**
+ * The language a client registers for a language entry of the index (shared/i18n.js registerLangs).
+ * @param {Record<string, any>} e a 'lang' entry of readPackIndex
+ * @returns {import('./i18nPacks.js').LangMeta}
+ */
+export function langMetaOf(e) {
+  const f = langFields(e.lang, e);
+  return {
+    ...f,
+    code: e.lang,
+    name: str(e.name, 60) || languageName(e.lang),
+    englishName: str(e.englishName, 60) || languageName(e.lang, 'en'),
+    authors: Array.isArray(e.authors) ? e.authors.map((a) => str(a)).filter(Boolean).slice(0, 32) : [],
+    version: str(e.version, 40),
+    app: str(e.app, 60),
+    compatible: e.compatible !== false,
+    strings: Number.isFinite(e.strings) ? e.strings : undefined,
+    data: !!e.files?.data,
+    ui: e.files?.ui,
+    dataUrl: e.files?.data,
+    packId: e.id,
+  };
+}