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src/i18n/index.tsx

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+import React, { createContext, useCallback, useContext, useEffect, useMemo, useState } from 'react';
+import { I18n } from 'i18n-js';
+import { getLocales } from 'expo-localization';
+import * as SecureStore from 'expo-secure-store';
+import en from './en';
+import it from './it';
+
+// English and Italian. Follows the phone's locale unless overridden in Profile.
+
+export type AppLanguage = 'en' | 'it';
+/** 'system' means follow the phone. */
+export type LanguagePreference = AppLanguage | 'system';
+
+export const SUPPORTED_LANGUAGES: AppLanguage[] = ['en', 'it'];
+
+const PREF_KEY = 'casadoc.language';
+
+const i18n = new I18n({ en, it });
+i18n.defaultLocale = 'en';
+i18n.enableFallback = true;
+
+/** Resolve the phone's locale to a language we actually ship. */
+export function deviceLanguage(): AppLanguage {
+  const tags = getLocales();
+  for (const l of tags) {
+    const code = (l.languageCode ?? '').toLowerCase();
+    if (code === 'it') return 'it';
+    if (code === 'en') return 'en';
+  }
+  return 'en'; // neither Italian nor English → English
+}
+
+function resolve(pref: LanguagePreference): AppLanguage {
+  return pref === 'system' ? deviceLanguage() : pref;
+}
+
+// Applied immediately, so translation calls before the provider mounts work.
+i18n.locale = deviceLanguage();
+
+/** For non-React code; inside components use `useT()` so it re-renders. */
+export function t(key: string, options?: Record<string, unknown>): string {
+  return i18n.t(key, options as any);
+}
+
+/** Current BCP-47-ish locale, for Intl date/number formatting. */
+export function currentLocale(): string {
+  return i18n.locale === 'it' ? 'it-IT' : 'en-GB';
+}
+
+interface I18nState {
+  language: AppLanguage;
+  preference: LanguagePreference;
+  setPreference: (pref: LanguagePreference) => void;
+  t: (key: string, options?: Record<string, unknown>) => string;
+}
+
+const I18nContext = createContext<I18nState | undefined>(undefined);
+
+export function I18nProvider({ children }: { children: React.ReactNode }) {
+  const [preference, setPreferenceState] = useState<LanguagePreference>('system');
+  const [ready, setReady] = useState(false);
+
+  useEffect(() => {
+    (async () => {
+      try {
+        const saved = await SecureStore.getItemAsync(PREF_KEY);
+        if (saved === 'en' || saved === 'it' || saved === 'system') {
+          setPreferenceState(saved);
+          i18n.locale = resolve(saved);
+        }
+      } catch {
+        // fall back to the system language
+      } finally {
+        setReady(true);
+      }
+    })();
+  }, []);
+
+  const setPreference = useCallback((pref: LanguagePreference) => {
+    setPreferenceState(pref);
+    i18n.locale = resolve(pref);
+    SecureStore.setItemAsync(PREF_KEY, pref).catch(() => {
+      // a lost preference only means we fall back to the phone's language
+    });
+  }, []);
+
+  const language = resolve(preference);
+
+  const value = useMemo<I18nState>(
+    () => ({
+      language,
+      preference,
+      setPreference,
+      t: (key: string, options?: Record<string, unknown>) => {
+        i18n.locale = language;
+        return i18n.t(key, options as any);
+      },
+    }),
+    [language, preference, setPreference],
+  );
+
+  // Avoids a flash of the wrong language before the preference loads.
+  if (!ready) return null;
+
+  return <I18nContext.Provider value={value}>{children}</I18nContext.Provider>;
+}
+
+export function useI18n(): I18nState {
+  const ctx = useContext(I18nContext);
+  if (!ctx) throw new Error('useI18n must be used within <I18nProvider>');
+  return ctx;
+}
+
+export function useT() {
+  return useI18n().t;
+}