diff --git a/client/src/api/client.ts b/client/src/api/client.ts
index 31ab75811..9502fda6c 100644
--- a/client/src/api/client.ts
+++ b/client/src/api/client.ts
@@ -126,7 +126,7 @@ const RATE_LIMIT_MESSAGES: Record<string, string> = {
 function translateRateLimit(): string {
   const fallback = RATE_LIMIT_MESSAGES['en']!
   try {
-    const lang = localStorage.getItem('app_language') || 'en'
+    const lang = localStorage.getItem('app_language') || localStorage.getItem('app_language_server') || 'en'
     return RATE_LIMIT_MESSAGES[lang] ?? fallback
   } catch {
     return fallback
diff --git a/client/src/store/authStore.ts b/client/src/store/authStore.ts
index f9960e8bc..532f9420a 100644
--- a/client/src/store/authStore.ts
+++ b/client/src/store/authStore.ts
@@ -12,6 +12,7 @@ import { useSystemNoticeStore } from './systemNoticeStore.js'
 import { clearAppearanceSnapshot } from '../theme/applyAppearance'
 import { clearAllPluginSessions } from './pluginStore'
 import { forgetStartDestination } from '../utils/startDestination'
+import { forgetServerLanguage } from './settingsStore'
 import { markSignedOut, clearSignedOut } from '../utils/signedOut'
 
 interface AuthResponse {
@@ -236,6 +237,10 @@ export const useAuthStore = create<AuthState>()(
     // And the startup-destination mirror, or the next account on this browser
     // gets bounced into a trip it may not even be able to see.
     forgetStartDestination()
+    // Likewise the language mirror: the login page only overrides it when the
+    // browser language is one TREK ships, so otherwise the next user here stays
+    // in the previous account's language, launch after launch.
+    forgetServerLanguage()
     // 4. Tell server to clear the httpOnly cookie (best-effort).
     await fetch('/api/auth/logout', { method: 'POST', credentials: 'include' }).catch(() => {})
     // 5. Clear service worker caches containing sensitive data.
diff --git a/client/src/store/settingsStore.test.ts b/client/src/store/settingsStore.test.ts
index e25ac3d9f..5e7ef67ba 100644
--- a/client/src/store/settingsStore.test.ts
+++ b/client/src/store/settingsStore.test.ts
@@ -1,5 +1,5 @@
 import { describe, it, expect, beforeEach, vi } from 'vitest'
-import { DEFAULT_SETTINGS, useSettingsStore } from './settingsStore'
+import { DEFAULT_SETTINGS, forgetServerLanguage, useSettingsStore } from './settingsStore'
 import { settingsApi } from '../api/client'
 import { clearTileCache } from '../sync/tilePrefetcher'
 
@@ -128,4 +128,35 @@ describe('offline language fallback', () => {
     const fresh = await import('./settingsStore')
     expect(fresh.DEFAULT_SETTINGS.language).toBe('de')
   })
+
+  // The account of the user who just signed in decides, and an account without a
+  // language of its own sends no language key at all. Leaving the previous
+  // account's mirror standing is what stranded the next user on a shared browser.
+  it('SETTINGS-LANG-005: an account with no language of its own clears the mirror', async () => {
+    localStorage.setItem('app_language_server', 'ja')
+    vi.mocked(settingsApi.get).mockResolvedValue({ settings: { default_currency: 'EUR' } } as never)
+
+    await useSettingsStore.getState().loadSettings()
+
+    expect(localStorage.getItem('app_language_server')).toBeNull()
+  })
+
+  it('SETTINGS-LANG-006: a language TREK does not ship never reaches the mirror', async () => {
+    localStorage.setItem('app_language_server', 'ja')
+    vi.mocked(settingsApi.get).mockResolvedValue({ settings: { language: 'kl' } } as never)
+
+    await useSettingsStore.getState().loadSettings()
+
+    expect(localStorage.getItem('app_language_server')).toBeNull()
+  })
+
+  it('SETTINGS-LANG-007: forgetServerLanguage drops the mirror and leaves the explicit choice alone', () => {
+    localStorage.setItem('app_language', 'de')
+    localStorage.setItem('app_language_server', 'ja')
+
+    forgetServerLanguage()
+
+    expect(localStorage.getItem('app_language_server')).toBeNull()
+    expect(localStorage.getItem('app_language')).toBe('de')
+  })
 })
diff --git a/client/src/store/settingsStore.ts b/client/src/store/settingsStore.ts
index b55997490..1dee91433 100644
--- a/client/src/store/settingsStore.ts
+++ b/client/src/store/settingsStore.ts
@@ -24,6 +24,33 @@ interface SettingsState {
 export const hasStoredLanguage = (): boolean =>
   typeof localStorage !== 'undefined' && !!localStorage.getItem('app_language')
 
+const SERVER_LANGUAGE_KEY = 'app_language_server'
+
+// An account that never picked a language sends no language key at all
+// (getUserSettings returns only what was set), so a mirror left over from
+// whoever used this browser before has to go rather than survive the load.
+function rememberServerLanguage(settings: Partial<Settings>): void {
+  try {
+    const language = settings.language
+    if (language && SUPPORTED_LANGUAGE_CODES.includes(language)) {
+      localStorage.setItem(SERVER_LANGUAGE_KEY, language)
+    } else {
+      localStorage.removeItem(SERVER_LANGUAGE_KEY)
+    }
+  } catch {
+    // Private mode: the session still has the value in the store.
+  }
+}
+
+/** On logout, so the next account on this browser starts in its own language. */
+export function forgetServerLanguage(): void {
+  try {
+    localStorage.removeItem(SERVER_LANGUAGE_KEY)
+  } catch {
+    // Nothing to clean up if storage is unavailable.
+  }
+}
+
 // The effective client-side defaults for a fresh instance. The server sends no value for
 // a setting an admin hasn't defaulted (see settingsService.getAdminUserDefaults), so these
 // are what a brand-new user actually sees. Keep them internally consistent — one
@@ -39,7 +66,7 @@ export const DEFAULT_SETTINGS: Settings = {
   // language (written by loadSettings below), then English. Without the mirror
   // an offline cold start boots in English: the account's language lives on the
   // server, and the fetch that would apply it cannot happen.
-  language: localStorage.getItem('app_language') || localStorage.getItem('app_language_server') || 'en',
+  language: localStorage.getItem('app_language') || localStorage.getItem(SERVER_LANGUAGE_KEY) || 'en',
   temperature_unit: 'celsius',
   distance_unit: 'metric',
   time_format: '24h',
@@ -110,9 +137,7 @@ export const useSettingsStore = create<SettingsState>((set, get) => ({
         // key rather than 'app_language': that one means an explicit in-app
         // choice, and the login page's detection chain must keep running for
         // users who never made one.
-        try {
-          if (incoming.language) localStorage.setItem('app_language_server', incoming.language)
-        } catch { /* private mode — the session still has the value in the store */ }
+        rememberServerLanguage(incoming)
       } catch (err: unknown) {
         // Leave isLoaded false so a transient failure — offline at launch, or a
         // (docker) server cold-start racing the first request — is retried on
diff --git a/client/tests/unit/stores/authStore.test.ts b/client/tests/unit/stores/authStore.test.ts
index 9b81d1083..1b556f86b 100644
--- a/client/tests/unit/stores/authStore.test.ts
+++ b/client/tests/unit/stores/authStore.test.ts
@@ -759,4 +759,20 @@ describe('authStore', () => {
       expect(syncTriggers.registerSyncTriggers).toHaveBeenCalled();
     });
   });
+
+  // The mirror of the account's server-side language is a per-device copy like the
+  // appearance snapshot and the startup destination, and gets dropped with them.
+  // 'app_language' is not a mirror but this device's own choice, so it stays.
+  describe('FE-STORE-AUTH-034: logout drops the language mirror', () => {
+    it('clears the account language but keeps an explicit in-app choice', async () => {
+      localStorage.setItem('app_language_server', 'ja');
+      localStorage.setItem('app_language', 'de');
+      useAuthStore.setState({ user: buildUser(), isAuthenticated: true });
+
+      await useAuthStore.getState().logout();
+
+      expect(localStorage.getItem('app_language_server')).toBeNull();
+      expect(localStorage.getItem('app_language')).toBe('de');
+    });
+  });
 });