frontend/angular/src/app/features/host/host-shell.component.ts

@@ -1,11 +1,11 @@
 import { CommonModule } from '@angular/common';
-import { Component, OnInit } from '@angular/core';
+import { Component, OnDestroy, OnInit } from '@angular/core';
 import { FormsModule } from '@angular/forms';
 
 import { createApiClient } from '../../../../../src/api/client';
 import type { FinishGameResponse, ScoreboardResponse } from '../../../../../src/api/types';
 import { createVerticalSliceController } from '../../../../../src/spa/vertical-slice';
-import { clientHasNoAudioOutput, resolvePreferredLocale, t } from '../../lobby-i18n';
+import { clientHasNoAudioOutput, resolvePreferredLocale, subscribeToLocaleChanges, t } from '../../lobby-i18n';
 
 interface SessionDetail {
   session: { code: string; status: string; current_round: number };
@@ -64,7 +64,7 @@ type LeaderboardResponse = FinishGameResponse;
     </div>
   `,
 })
-export class HostShellComponent implements OnInit {
+export class HostShellComponent implements OnInit, OnDestroy {
   locale = resolvePreferredLocale();
   readonly clientHasNoAudioOutput = clientHasNoAudioOutput;
 
@@ -84,8 +84,12 @@ export class HostShellComponent implements OnInit {
 
   private readonly api = createApiClient();
   private readonly controller = createVerticalSliceController(this.api);
+  private unsubscribeLocale: (() => void) | null = null;
 
   ngOnInit(): void {
+    this.unsubscribeLocale = subscribeToLocaleChanges((locale) => {
+      this.locale = locale;
+    });
     if (typeof window === 'undefined') {
       return;
     }
@@ -105,6 +109,11 @@ export class HostShellComponent implements OnInit {
     void this.refreshSession();
   }
 
+  ngOnDestroy(): void {
+    this.unsubscribeLocale?.();
+    this.unsubscribeLocale = null;
+  }
+
   copy(key: string): string {
     return t(key, this.locale);
   }

frontend/angular/src/app/features/player/player-shell.component.ts

@@ -5,7 +5,7 @@ import { FormsModule } from '@angular/forms';
 import { createApiClient } from '../../../../../src/api/client';
 import { createSessionContextStore } from '../../../../../src/spa/session-context-store';
 import { createVerticalSliceController } from '../../../../../src/spa/vertical-slice';
-import { clientHasNoAudioOutput, resolvePreferredLocale, t } from '../../lobby-i18n';
+import { clientHasNoAudioOutput, resolvePreferredLocale, subscribeToLocaleChanges, t } from '../../lobby-i18n';
 
 interface SessionDetail {
   session: { code: string; status: string; current_round: number };
@@ -112,6 +112,7 @@ export class PlayerShellComponent implements OnInit, OnDestroy {
   private readonly controller = createVerticalSliceController(createApiClient(), this.sessionContextStore);
   private reconnectTimer: ReturnType<typeof setTimeout> | null = null;
   private stateSyncTimer: ReturnType<typeof setTimeout> | null = null;
+  private unsubscribeLocale: (() => void) | null = null;
 
   constructor() {
     if (typeof navigator !== 'undefined' && !navigator.onLine) {
@@ -125,6 +126,10 @@ export class PlayerShellComponent implements OnInit, OnDestroy {
   }
 
   ngOnInit(): void {
+    this.unsubscribeLocale = subscribeToLocaleChanges((locale) => {
+      this.locale = locale;
+    });
+
     const hashRoute = window.location.hash.replace(/^#\/?/, '');
     const match = hashRoute.match(/^player(?:\/[^/]+)?(?:\/([^/?#]+))?/i);
     const codeFromRoute = match?.[1] ?? '';
@@ -151,6 +156,8 @@ export class PlayerShellComponent implements OnInit, OnDestroy {
     }
     this.clearReconnectTimer();
     this.clearStateSyncTimer();
+    this.unsubscribeLocale?.();
+    this.unsubscribeLocale = null;
   }
 
   private readonly handleOnline = (): void => {

frontend/angular/src/app/lobby-i18n.spec.ts

@@ -0,0 +1,47 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+type StorageLike = {
+  getItem: (key: string) => string | null;
+  setItem: (key: string, value: string) => void;
+};
+
+function storageMock(initial: Record<string, string> = {}): StorageLike {
+  const data = new Map<string, string>(Object.entries(initial));
+  return {
+    getItem: vi.fn((key: string) => data.get(key) ?? null),
+    setItem: vi.fn((key: string, value: string) => {
+      data.set(key, value);
+    }),
+  };
+}
+
+describe('lobby i18n locale propagation', () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+    vi.unstubAllGlobals();
+    vi.resetModules();
+  });
+
+  it('notifies subscribers immediately and on locale changes', async () => {
+    const localStorage = storageMock({ 'wpp.locale': 'en' });
+    vi.stubGlobal('window', {
+      location: { search: '' },
+      localStorage,
+    });
+    vi.stubGlobal('navigator', { language: 'en-US' });
+
+    const i18n = await import('./lobby-i18n');
+
+    const updates: string[] = [];
+    const unsubscribe = i18n.subscribeToLocaleChanges((locale) => updates.push(locale));
+
+    expect(updates).toEqual(['en']);
+
+    i18n.setPreferredLocale('da');
+    expect(updates).toEqual(['en', 'da']);
+
+    unsubscribe();
+    i18n.setPreferredLocale('en');
+    expect(updates).toEqual(['en', 'da']);
+  });
+});

frontend/angular/src/app/lobby-i18n.ts

@@ -5,6 +5,9 @@ type SupportedLocale = (typeof lobbyCatalog.locales.supported)[number];
 const DEFAULT_LOCALE = lobbyCatalog.locales.default as SupportedLocale;
 const SUPPORTED_LOCALES = lobbyCatalog.locales.supported as readonly SupportedLocale[];
 
+let activeLocale: SupportedLocale | null = null;
+const localeSubscribers = new Set<(locale: SupportedLocale) => void>();
+
 export function normalizeLocale(rawLocale?: string | null): SupportedLocale {
   const locale = (rawLocale ?? '').trim().toLowerCase();
   if ((SUPPORTED_LOCALES as readonly string[]).includes(locale)) {
@@ -20,25 +23,45 @@ export function normalizeLocale(rawLocale?: string | null): SupportedLocale {
 }
 
 export function resolvePreferredLocale(): SupportedLocale {
+  if (activeLocale) {
+    return activeLocale;
+  }
+
   if (typeof window === 'undefined') {
-    return DEFAULT_LOCALE;
+    activeLocale = DEFAULT_LOCALE;
+    return activeLocale;
   }
 
   const queryLocale = new URLSearchParams(window.location?.search ?? '').get('lang');
   const storedLocale = window.localStorage?.getItem?.('wpp.locale');
   const browserLocale = typeof navigator !== 'undefined' ? navigator.language : '';
 
-  return normalizeLocale(queryLocale || storedLocale || browserLocale || DEFAULT_LOCALE);
+  activeLocale = normalizeLocale(queryLocale || storedLocale || browserLocale || DEFAULT_LOCALE);
+  return activeLocale;
 }
 
 export function setPreferredLocale(locale: string): SupportedLocale {
   const normalized = normalizeLocale(locale);
+  activeLocale = normalized;
   if (typeof window !== 'undefined') {
     window.localStorage?.setItem?.('wpp.locale', normalized);
   }
+
+  for (const subscriber of localeSubscribers) {
+    subscriber(normalized);
+  }
+
   return normalized;
 }
 
+export function subscribeToLocaleChanges(callback: (locale: SupportedLocale) => void): () => void {
+  localeSubscribers.add(callback);
+  callback(resolvePreferredLocale());
+  return () => {
+    localeSubscribers.delete(callback);
+  };
+}
+
 export function t(key: string, locale: string): string {
   const normalizedLocale = normalizeLocale(locale);
   const fallbackLocale = DEFAULT_LOCALE;