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.

test/ui/i18n.e2e.test.js

@@ -4,15 +4,25 @@
 //
 // The title screen opens in Chinese; 中文 | English → English in place (no reload): the title, the start button, the
 // connection line, <html lang>, the tab title; the choice is kept across a reload (localStorage sp.pref.lang) and the
-// game-data overlay (data/i18n/en.json) is applied; `?lang=zh` switches back and leaves the address bar. No console /
-// page / request errors. docs/I18N.md.
+// game-data overlay (data/i18n/en.json) is applied; `?lang=zh` switches back and leaves the address bar. Language packs
+// (docs/I18N.md "Adding a language", docs/PACKS.md): a pack file dropped into public/i18n/ and a pack folder dropped
+// into packs/ while the server runs show in the menu and switch, with English filling what they lack; a removed pack
+// sends a stored choice back to Chinese. No console / page / request errors. docs/I18N.md.
 
 import { test, describe, before, after } from 'node:test';
 import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
 import { CHROME, hasChrome, sleep } from '../e2e/client.mjs';
 import { NAME_MAX_LEN } from '../../shared/constants.js';
 
 const ENABLED = process.env.SP_E2E === '1' && hasChrome();
+const ROOT = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '../..');
+// example packs, only for this test ('qaa' … 'qtz' are the ISO 639 codes reserved for local use); removed afterwards
+const PACK_FILE = path.join(ROOT, 'public/i18n/qaa.json');
+const PACK_DIR = path.join(ROOT, 'packs/qab');
+const removePacks = () => { fs.rmSync(PACK_FILE, { force: true }); fs.rmSync(PACK_DIR, { recursive: true, force: true }); };
 
 describe('language switch on the title screen', { skip: !ENABLED && 'set SP_E2E=1 (Chrome)' }, () => {
   let srv;
@@ -25,7 +35,7 @@ describe('language switch on the title screen', { skip: !ENABLED && 'set SP_E2E=
     base = `http://127.0.0.1:${srv.port}`;
     browser = await puppeteer.launch({ executablePath: CHROME, headless: true, args: ['--no-sandbox', '--force-device-scale-factor=1'] });
   });
-  after(async () => { await browser?.close(); await srv?.close(); });
+  after(async () => { removePacks(); await browser?.close(); await srv?.close(); });
 
   const text = (page, sel) => page.$eval(sel, (el) => el.textContent.replace(/\s+/g, ' ').trim());
 
@@ -69,4 +79,50 @@ describe('language switch on the title screen', { skip: !ENABLED && 'set SP_E2E=
     assert.deepEqual(problems, []);
     await page.close();
   });
+  test('a pack dropped into the language folder or into packs/ shows in the menu and switches; English fills its gaps', async () => {
+    const page = await browser.newPage();
+    await page.setViewport({ width: 1600, height: 900 });
+    const problems = [];
+    page.on('console', (m) => { if (m.type() === 'error') problems.push(`console: ${m.text()}`); });
+    page.on('pageerror', (e) => problems.push(`pageerror: ${e.message}`));
+    page.on('response', (r) => { if (r.status() >= 400 && !/fonts\.(googleapis|gstatic)/.test(r.url())) problems.push(`http ${r.status()}: ${r.url()}`); });
+    try {
+      fs.writeFileSync(PACK_FILE, JSON.stringify({ _meta: { name: 'Testisch', englishName: 'Test language', fallback: ['en'] }, 卫戍协议: 'Testprotokoll', 盟约: 'Bund', 开始: 'Los' }));
+      fs.mkdirSync(PACK_DIR, { recursive: true });
+      fs.writeFileSync(path.join(PACK_DIR, 'pack.json'), JSON.stringify({ type: 'lang', lang: 'qab', name: 'Qabisch', files: { ui: 'ui.json' } }));
+      fs.writeFileSync(path.join(PACK_DIR, 'ui.json'), JSON.stringify({ 开始: 'Qab-Start' }));
+      await sleep(1100); // the server's registry looks at the folders again after a second
+      await page.goto(`${base}/?lang=zh`, { waitUntil: 'networkidle0' });
+      await page.waitForSelector('.title-screen [data-testid="lang-toggle"] button[data-lang="qaa"]');
+      assert.deepEqual(await page.$$eval('[data-testid="lang-toggle"] button', (bs) => bs.map((b) => [b.dataset.lang, b.textContent.trim()])),
+        [['zh', '中文'], ['en', 'English'], ['qaa', 'Testisch'], ['qab', 'Qabisch']]);
+
+      await page.click('[data-testid="lang-toggle"] button[data-lang="qaa"]');
+      await page.waitForFunction(() => document.querySelector('.title-login .btn--primary')?.textContent.includes('Los'), { timeout: 8000 });
+      assert.equal(await text(page, '.title-cn'), 'Testprotokoll: Bund', 'an alphabetic title: the display face, an ASCII colon');
+      assert.match(await text(page, '.title-tag'), /^Allocate Funds and Operators/, 'untranslated: English, the pack\'s fallback');
+      assert.deepEqual(await page.evaluate(() => ({ lang: document.documentElement.lang, script: document.documentElement.dataset.script, pref: localStorage.getItem('sp.pref.lang') })),
+        { lang: 'qaa', script: 'alphabetic', pref: '"qaa"' });
+      // no game texts of its own: English's through the chain
+      await page.waitForFunction(() => globalThis.__SP__?.data?.locale() === 'qaa', { timeout: 8000 });
+      assert.equal(await page.evaluate(() => globalThis.__SP__.data.get('config').modes.mode_multi_abyss.name), 'Ultimate Simulation');
+
+      await page.click('[data-testid="lang-toggle"] button[data-lang="qab"]');
+      await page.waitForFunction(() => document.querySelector('.title-login .btn--primary')?.textContent.includes('Qab-Start'), { timeout: 8000 });
+      assert.equal(await text(page, '.title-cn'), '卫戍协议：盟约', 'no fallback declared: the Chinese msgid');
+
+      await page.click('[data-testid="lang-toggle"] button[data-lang="qaa"]');
+      await page.waitForFunction(() => document.querySelector('.title-login .btn--primary')?.textContent.includes('Los'), { timeout: 8000 });
+      removePacks();
+      await sleep(1100);
+      await page.reload({ waitUntil: 'networkidle0' });
+      await page.waitForSelector('.title-screen .title-cn');
+      assert.equal(await text(page, '.title-cn'), '卫戍协议：盟约', 'the stored pack is gone: Chinese');
+      assert.deepEqual(await page.$$eval('[data-testid="lang-toggle"] button', (bs) => bs.map((b) => b.dataset.lang)), ['zh', 'en']);
+      assert.deepEqual(problems, []);
+    } finally {
+      removePacks();
+      await page.close();
+    }
+  });
 });