diff --git a/public/js/ui/lang.js b/public/js/ui/lang.js
index 5f95bfbf..644318b5 100644
--- a/public/js/ui/lang.js
+++ b/public/js/ui/lang.js
@@ -39,6 +39,8 @@ const uiLoads = new Map();
 let indexLoad = null;
 /** Whether the pack index arrived (without it the chain of a pack is taken from its own `_meta` as it loads). */
 let indexOk = false;
+/** Only the latest boot or explicit selection may apply its loaded language. */
+let langRequest = 0;
 const INDEX_URL = `${PACKS_URL}${PACK_INDEX_FILE}`;
 
 const defaultFetch = (...a) => globalThis.fetch(...a);
@@ -194,20 +196,23 @@ function applyLang(lang) {
  * @returns {Promise<string>} the language in effect
  */
 export async function initLang() {
+  const request = ++langRequest;
   wire();
   applyDocument(getLang());
   const TIMEOUT = Symbol('timeout');
   const boot = (async () => {
     const listed = await loadLangIndex();
     const { lang, fromUrl } = initialLang(undefined, undefined, { tentative: !listed });
-    if (fromUrl) { savePref(PREF_KEY, lang); stripLangParam(); }
+    if (fromUrl) stripLangParam();
+    if (request !== langRequest) return null;
+    if (fromUrl) savePref(PREF_KEY, lang);
     if (lang === DEFAULT_LANG) return null;
     return (await loadLangChain(lang)) ? lang : null;
   })();
   const got = await Promise.race([boot, new Promise((r) => setTimeout(() => r(TIMEOUT), BOOT_WAIT_MS))]);
   if (got === TIMEOUT) {
-    boot.then((lang) => { if (lang && normalizeLang(loadPref(PREF_KEY, null)) === lang) applyLang(lang); });
-  } else if (got) applyLang(got);
+    boot.then((lang) => { if (request === langRequest && lang && normalizeLang(loadPref(PREF_KEY, null)) === lang) applyLang(lang); });
+  } else if (request === langRequest && got) applyLang(got);
   return getLang();
 }
 
@@ -218,10 +223,12 @@ export async function initLang() {
  * @returns {Promise<string>} the language in effect
  */
 export async function switchLang(lang) {
+  const request = ++langRequest;
   wire();
   const want = normalizeLang(lang) || DEFAULT_LANG;
   savePref(PREF_KEY, want);
   if (want !== DEFAULT_LANG && !(await loadLangChain(want))) return getLang();
+  if (request !== langRequest) return getLang();
   applyLang(want);
   return getLang();
 }
@@ -290,10 +297,17 @@ export function machineTranslationNote() {
 export function LangToggle({ class: cls }) {
   const lang = useLang();
   const menu = langMenuModel(useLangs(), lang);
-  const pick = (code) => { if (code && code !== getLang()) switchLang(code); };
+  const pick = async (code, select = null) => {
+    if (!code) return;
+    const switched = switchLang(code);
+    const request = langRequest;
+    await switched;
+    // A failed current request keeps the displayed language; stale requests must not reset a newer pending pick.
+    if (select && request === langRequest) select.value = getLang();
+  };
   if (menu.kind === 'select') {
     return html`<label class=${`set-seg lang-toggle lang-select${cls ? ` ${cls}` : ''}`} data-testid="lang-toggle">
-      <select aria-label=${SWITCH_LABEL} value=${lang} onChange=${(e) => pick(e.currentTarget.value)}>
+      <select aria-label=${SWITCH_LABEL} value=${lang} onChange=${(e) => pick(e.currentTarget.value, e.currentTarget)}>
         ${menu.items.map((it) => html`<option key=${it.code} value=${it.code} lang=${it.htmlLang} title=${it.title || undefined} selected=${it.on}>${it.label}</option>`)}
       </select>
     </label>`;
diff --git a/test/ui/i18n.e2e.test.js b/test/ui/i18n.e2e.test.js
index dfd76425..eb955237 100644
--- a/test/ui/i18n.e2e.test.js
+++ b/test/ui/i18n.e2e.test.js
@@ -179,4 +179,125 @@ describe('language switch on the title screen', { skip: !ENABLED && 'set SP_E2E=
     assert.deepEqual(problems, []);
     await page.close();
   });
+
+  // Hold only UI pack fetches; the real loader, cache, Preact menu and document updates still run.
+  const holdPacks = (page, codes) => page.evaluate((held) => {
+    const fetch = globalThis.fetch.bind(globalThis);
+    globalThis.languageRequests = {};
+    const gates = Object.fromEntries(held.map((code) => {
+      let release;
+      const ready = new Promise((resolve) => { release = resolve; });
+      return [code, { ready, release }];
+    }));
+    globalThis.releaseLanguage = async (code, ok) => {
+      const { loadLangChain } = await import('/js/ui/lang.js');
+      const settled = loadLangChain(code);
+      gates[code].release(ok);
+      await settled;
+      await new Promise(requestAnimationFrame);
+    };
+    globalThis.fetch = (url, ...args) => {
+      const code = held.find((c) => url === `/i18n/${c}.json`);
+      if (!code) return fetch(url, ...args);
+      globalThis.languageRequests[code] = (globalThis.languageRequests[code] || 0) + 1;
+      return gates[code].ready.then((ok) => ok ? fetch(url, ...args) : new Response('', { status: 503 }));
+    };
+  }, codes);
+  const waitForPack = (page, code) => page.waitForFunction((c) => globalThis.languageRequests[c] > 0, {}, code);
+  const releasePack = (page, code, ok = true) => page.evaluate((c, success) => globalThis.releaseLanguage(c, success), code, ok);
+  const languageState = (page) => page.evaluate(async () => ({
+    current: (await import('/shared/i18n.js')).getLang(),
+    document: document.documentElement.lang,
+    dataLang: document.documentElement.dataset.lang,
+    script: document.documentElement.dataset.script,
+    title: document.title,
+    start: document.querySelector('.title-login .btn--primary').textContent.trim(),
+    selected: document.querySelector('[data-testid="lang-toggle"] select').value,
+    saved: JSON.parse(localStorage.getItem('sp.pref.lang')),
+  }));
+  const expectedLanguage = {
+    zh: { current: 'zh', document: 'zh-CN', dataLang: 'zh', script: 'cjk', title: '卫戍协议：盟约 · STRONGHOLD PROTOCOL', start: '开始', selected: 'zh', saved: 'zh' },
+    en: { current: 'en', document: 'en', dataLang: 'en', script: 'alphabetic', title: 'Stronghold Protocol: Alliance · Web Simulation', start: 'Start', selected: 'en', saved: 'en' },
+    'zh-TW': { current: 'zh-TW', document: 'zh-TW', dataLang: 'zh-TW', script: 'cjk', title: '衛戍協議：盟約 · STRONGHOLD PROTOCOL', start: '開始', selected: 'zh-TW', saved: 'zh-TW' },
+  };
+
+  test('a late English pack cannot overwrite a newer Traditional Chinese selection; its cache remains reusable', async (t) => {
+    const page = await browser.newPage();
+    t.after(() => page.close());
+    await page.goto(`${base}/?lang=zh`, { waitUntil: 'networkidle0' });
+    await holdPacks(page, ['en']);
+    await pick(page, 'en');
+    await waitForPack(page, 'en');
+    await pick(page, 'zh-TW');
+    await page.waitForFunction(() => document.documentElement.lang === 'zh-TW');
+    await releasePack(page, 'en');
+    assert.deepEqual(await languageState(page), expectedLanguage['zh-TW']);
+    await pick(page, 'en');
+    await page.waitForFunction(() => document.documentElement.lang === 'en');
+    assert.deepEqual(await languageState(page), expectedLanguage.en);
+    assert.equal(await page.evaluate(() => globalThis.languageRequests.en), 1, 'the stale pack was cached');
+  });
+
+  test('selecting the displayed default language cancels a pending pack', async (t) => {
+    const page = await browser.newPage();
+    t.after(() => page.close());
+    await page.goto(`${base}/?lang=zh`, { waitUntil: 'networkidle0' });
+    await holdPacks(page, ['en']);
+    await pick(page, 'en');
+    await waitForPack(page, 'en');
+    assert.equal(await page.$eval('[data-testid="lang-toggle"] select', (el) => el.value), 'en', 'the native menu can still change back to Chinese');
+    assert.equal(await page.$eval('[data-testid="lang-toggle"] select', (el) => el.disabled), false);
+    await pick(page, 'zh');
+    await releasePack(page, 'en');
+    assert.deepEqual(await languageState(page), expectedLanguage.zh);
+  });
+
+  test('repeated selections reuse a pending pack and still supersede an intervening selection', async (t) => {
+    const page = await browser.newPage();
+    t.after(() => page.close());
+    await page.goto(`${base}/?lang=zh`, { waitUntil: 'networkidle0' });
+    await holdPacks(page, ['en', 'zh-TW']);
+    await pick(page, 'en');
+    await waitForPack(page, 'en');
+    await pick(page, 'en');
+    await pick(page, 'zh-TW');
+    await waitForPack(page, 'zh-TW');
+    await pick(page, 'en');
+    await releasePack(page, 'en');
+    assert.deepEqual(await languageState(page), expectedLanguage.en);
+    await releasePack(page, 'zh-TW');
+    assert.deepEqual(await languageState(page), expectedLanguage.en);
+    assert.equal(await page.evaluate(() => globalThis.languageRequests.en), 1);
+  });
+
+  test('a failed latest selection keeps the last successful UI and menu; an older pack stays stale', async (t) => {
+    const page = await browser.newPage();
+    t.after(() => page.close());
+    await page.goto(`${base}/?lang=zh`, { waitUntil: 'networkidle0' });
+    await holdPacks(page, ['en', 'zh-TW']);
+    await pick(page, 'en');
+    await waitForPack(page, 'en');
+    await pick(page, 'zh-TW');
+    await waitForPack(page, 'zh-TW');
+    await releasePack(page, 'zh-TW', false);
+    const unchanged = { ...expectedLanguage.zh, saved: 'zh-TW' };
+    assert.deepEqual(await languageState(page), unchanged, 'failed preference stays saved, as before');
+    await releasePack(page, 'en');
+    assert.deepEqual(await languageState(page), unchanged);
+  });
+
+  test('a stale failure leaves a newer pending dropdown selection intact', async (t) => {
+    const page = await browser.newPage();
+    t.after(() => page.close());
+    await page.goto(`${base}/?lang=zh`, { waitUntil: 'networkidle0' });
+    await holdPacks(page, ['en', 'zh-TW']);
+    await pick(page, 'en');
+    await waitForPack(page, 'en');
+    await pick(page, 'zh-TW');
+    await waitForPack(page, 'zh-TW');
+    await releasePack(page, 'en', false);
+    assert.deepEqual(await languageState(page), { ...expectedLanguage.zh, saved: 'zh-TW', selected: 'zh-TW' });
+    await releasePack(page, 'zh-TW');
+    assert.deepEqual(await languageState(page), expectedLanguage['zh-TW']);
+  });
 });
diff --git a/test/ui/lang.test.js b/test/ui/lang.test.js
new file mode 100644
index 00000000..ebbfd7b8
--- /dev/null
+++ b/test/ui/lang.test.js
@@ -0,0 +1,222 @@
+// Controlled UI-pack completions exercise real language switching without network timing or a browser.
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { getLang, onLangChange, registerLangs, setLang, setMessages, setI18nWarn, t } from '../../shared/i18n.js';
+
+const TITLE = '卫戍协议：盟约 · STRONGHOLD PROTOCOL';
+const PACKS = {
+  en: { 开始: 'Start', 卫戍协议: 'Stronghold Protocol', [TITLE]: 'Stronghold Protocol: Alliance' },
+  'zh-TW': { 开始: '開始', 卫戍协议: '衛戍協議', [TITLE]: '衛戍協議：盟約' },
+};
+const STATES = {
+  zh: { lang: 'zh', text: '开始', htmlLang: 'zh-CN', dataLang: 'zh', script: 'cjk', title: TITLE },
+  en: { lang: 'en', text: 'Start', htmlLang: 'en', dataLang: 'en', script: 'alphabetic', title: 'Stronghold Protocol: Alliance' },
+  'zh-TW': { lang: 'zh-TW', text: '開始', htmlLang: 'zh-TW', dataLang: 'zh-TW', script: 'cjk', title: '衛戍協議：盟約' },
+};
+const INDEX = { version: 1, packs: Object.keys(PACKS).map((code) => ({
+  id: code, type: 'lang', lang: code, name: code, files: { ui: `/i18n/${code}.json` },
+})) };
+const turn = () => new Promise((resolve) => setImmediate(resolve));
+let imports = 0;
+
+async function setup(ctx, preference = null, address = 'https://example.test/') {
+  const previous = new Map(['document', 'localStorage', 'fetch', 'location', 'history'].map((key) => [key, Object.getOwnPropertyDescriptor(globalThis, key)]));
+  const stored = new Map(preference ? [['sp.pref.lang', JSON.stringify(preference)]] : []);
+  const doc = { documentElement: { lang: 'zh-CN', dataset: { lang: 'zh', script: 'cjk' } }, title: TITLE };
+  const pending = new Map();
+  const calls = [];
+  globalThis.document = doc;
+  globalThis.localStorage = { getItem: (key) => stored.get(key) ?? null, setItem: (key, value) => stored.set(key, value) };
+  globalThis.location = new URL(address);
+  globalThis.history = {
+    state: null,
+    replaceState(state, _title, relative) {
+      this.state = state;
+      globalThis.location.href = new URL(relative, globalThis.location.href).href;
+    },
+  };
+  globalThis.fetch = (url) => {
+    calls.push(url);
+    const request = Promise.withResolvers();
+    if (!pending.has(url)) pending.set(url, []);
+    pending.get(url).push(request);
+    return request.promise;
+  };
+  const warnings = [];
+  ctx.mock.method(console, 'warn', (message) => warnings.push(message));
+  registerLangs(Object.keys(PACKS).map((code) => ({ code, fallback: [], base: null, data: false, ui: `/i18n/${code}.json` })));
+  setLang('zh');
+  for (const code of Object.keys(PACKS)) setMessages(code, {});
+  const url = new URL('../../public/js/ui/lang.js', import.meta.url);
+  url.searchParams.set('test', String(++imports));
+  const lang = await import(url.href);
+  ctx.after(() => {
+    setLang('zh');
+    setI18nWarn(null);
+    for (const code of Object.keys(PACKS)) setMessages(code, {});
+    for (const [key, descriptor] of previous) {
+      if (descriptor) Object.defineProperty(globalThis, key, descriptor);
+      else delete globalThis[key];
+    }
+  });
+  const finish = (path, body, status = 200) => {
+    const request = pending.get(path)?.shift();
+    assert.ok(request, `a request is pending for ${path}`);
+    request.resolve({ ok: status === 200, status, json: async () => body });
+  };
+  const state = (code, saved = code) => assert.deepEqual({
+    lang: getLang(), text: t('开始'), htmlLang: doc.documentElement.lang, dataLang: doc.documentElement.dataset.lang,
+    script: doc.documentElement.dataset.script, title: doc.title, saved: stored.get('sp.pref.lang') ?? null,
+  }, { ...STATES[code], saved: saved == null ? null : JSON.stringify(saved) });
+  return {
+    ...lang, calls, warnings, state,
+    finish: (code, status = 200) => finish(`/i18n/${code}.json`, PACKS[code], status),
+    finishIndex: () => finish('/packs/index.json', INDEX),
+  };
+}
+
+test('a delayed language cannot overwrite the later successful selection', async (ctx) => {
+  const h = await setup(ctx);
+  const first = h.switchLang('en');
+  const latest = h.switchLang('zh-TW');
+  h.finish('zh-TW');
+  assert.equal(await latest, 'zh-TW');
+  h.state('zh-TW');
+  h.finish('en');
+  assert.equal(await first, 'zh-TW');
+  h.state('zh-TW');
+});
+
+test('selecting the default language invalidates a pending pack', async (ctx) => {
+  const h = await setup(ctx);
+  const first = h.switchLang('en');
+  assert.equal(await h.switchLang('zh'), 'zh');
+  h.state('zh');
+  h.finish('en');
+  assert.equal(await first, 'zh');
+  h.state('zh');
+});
+
+test('reselecting the current language cancels a different pending selection and uses its cached pack', async (ctx) => {
+  const h = await setup(ctx);
+  const initial = h.switchLang('zh-TW');
+  h.finish('zh-TW');
+  await initial;
+  const pending = h.switchLang('en');
+  await h.switchLang('zh-TW');
+  h.finish('en');
+  await pending;
+  h.state('zh-TW');
+  assert.deepEqual(h.calls, ['/i18n/zh-TW.json', '/i18n/en.json']);
+});
+
+test('A, B, A shares the in-flight A pack and applies only the latest selection', async (ctx) => {
+  const h = await setup(ctx);
+  const changes = [];
+  ctx.after(onLangChange((code) => changes.push(code)));
+  const first = h.switchLang('en');
+  const middle = h.switchLang('zh-TW');
+  const latest = h.switchLang('en');
+  h.finish('zh-TW');
+  await middle;
+  h.state('zh', 'en');
+  h.finish('en');
+  await Promise.all([first, latest]);
+  h.state('en');
+  assert.deepEqual(changes, ['en']);
+  await h.switchLang('en');
+  assert.deepEqual(h.calls, ['/i18n/en.json', '/i18n/zh-TW.json']);
+  assert.deepEqual(changes, ['en'], 'repeated successful selections do not notify a language change');
+});
+
+test('a failed latest request retains the successful interface, rejects stale success, and can retry', async (ctx) => {
+  const h = await setup(ctx);
+  const first = h.switchLang('en');
+  const latest = h.switchLang('zh-TW');
+  h.finish('zh-TW', 503);
+  assert.equal(await latest, 'zh');
+  h.state('zh', 'zh-TW');
+  assert.equal(h.warnings.length, 1);
+  assert.match(h.warnings[0], /HTTP 503.*interface stays as it is/);
+  h.finish('en');
+  await first;
+  h.state('zh', 'zh-TW');
+  const retry = h.switchLang('zh-TW');
+  h.finish('zh-TW');
+  await retry;
+  h.state('zh-TW');
+  await h.switchLang('en');
+  h.state('en');
+  assert.deepEqual(h.calls, ['/i18n/en.json', '/i18n/zh-TW.json', '/i18n/zh-TW.json'], 'failed loads retry; stale successful packs remain cached');
+});
+
+test('startup applies the stored language after its pack is ready', async (ctx) => {
+  const h = await setup(ctx, 'en');
+  ctx.mock.method(globalThis, 'setTimeout', () => 0);
+  const boot = h.initLang();
+  h.finishIndex();
+  await turn();
+  h.finish('en');
+  assert.equal(await boot, 'en');
+  h.state('en');
+});
+
+test('startup cannot overwrite a newer switch before the boot timeout', async (ctx) => {
+  const h = await setup(ctx, 'en');
+  ctx.mock.method(globalThis, 'setTimeout', () => 0);
+  const boot = h.initLang();
+  h.finishIndex();
+  await turn();
+  const latest = h.switchLang('zh-TW');
+  h.finish('zh-TW');
+  await latest;
+  h.finish('en');
+  await boot;
+  h.state('zh-TW');
+});
+
+test('a delayed startup index cannot restore the URL choice after a newer selection', async (ctx) => {
+  const h = await setup(ctx, null, 'https://example.test/play?room=alliance&lang=en#settings');
+  ctx.mock.method(globalThis, 'setTimeout', () => 0);
+  const boot = h.initLang();
+  const latest = h.switchLang('zh-TW');
+  h.finish('zh-TW');
+  await latest;
+  h.finishIndex();
+  await turn();
+  h.state('zh-TW');
+  assert.equal(await boot, 'zh-TW');
+  assert.deepEqual(h.calls, ['/packs/index.json', '/i18n/zh-TW.json']);
+  assert.equal(globalThis.location.href, 'https://example.test/play?room=alliance#settings');
+  assert.deepEqual(h.initialLang(), { lang: 'zh-TW', fromUrl: false }, 'reload uses the newer saved choice');
+});
+
+test('timed-out startup applies its pack later when the selection has not changed', async (ctx) => {
+  const h = await setup(ctx, 'en');
+  let timeout;
+  ctx.mock.method(globalThis, 'setTimeout', (fn) => { timeout = fn; return 0; });
+  const boot = h.initLang();
+  h.finishIndex();
+  await turn();
+  timeout();
+  assert.equal(await boot, 'zh');
+  h.state('zh', 'en');
+  h.finish('en');
+  await turn();
+  h.state('en');
+});
+
+test('timed-out startup cannot overwrite a newer default selection', async (ctx) => {
+  const h = await setup(ctx, 'en');
+  let timeout;
+  ctx.mock.method(globalThis, 'setTimeout', (fn) => { timeout = fn; return 0; });
+  const boot = h.initLang();
+  h.finishIndex();
+  await turn();
+  timeout();
+  await boot;
+  await h.switchLang('zh');
+  h.finish('en');
+  await turn();
+  h.state('zh');
+});