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apps/desktop/src/lib/i18n.ts

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+import {
+  DEFAULT_LOCALE,
+  detectBrowserLocale,
+  initI18n,
+  isSupportedLocale,
+  type SupportedLocale,
+} from '@chat-app/shared/i18n';
+
+const LOCAL_STORAGE_KEY = 'chatapp.locale';
+
+export function getCachedLocale(): SupportedLocale | null {
+  try {
+    const raw = window.localStorage.getItem(LOCAL_STORAGE_KEY);
+    return isSupportedLocale(raw) ? raw : null;
+  } catch {
+    return null;
+  }
+}
+
+export function cacheLocale(locale: SupportedLocale): void {
+  try {
+    window.localStorage.setItem(LOCAL_STORAGE_KEY, locale);
+  } catch {
+    // Ignore (private mode, quota, etc.). Profile row remains source of truth.
+  }
+}
+
+// Precedence for the very first paint (before we know the user):
+//   1. cached choice from a previous session
+//   2. navigator language(s)
+//   3. DEFAULT_LOCALE (en)
+export function resolveInitialLocale(): SupportedLocale {
+  return (
+    getCachedLocale() ??
+    detectBrowserLocale(typeof navigator !== 'undefined' ? navigator.languages : undefined) ??
+    DEFAULT_LOCALE
+  );
+}
+
+export function bootstrapI18n(): void {
+  const initialLocale = resolveInitialLocale();
+  initI18n({
+    initialLocale,
+    onLanguageChanged: (locale) => cacheLocale(locale),
+  });
+}