feature: localization's first implmentation

frontend/src/lib/hooks/useAuth.tsx

@@ -1,16 +1,17 @@
 'use client';
 
 import React, { createContext, useCallback, useContext, useEffect, useMemo, useState } from 'react';
-import { useRouter } from 'next/navigation';
+import { useRouter } from '@/i18n/navigation';
 import { authApi } from '@/lib/api/auth';
 import { User, Organization } from '@/types/organization';
+import { isAppLocale } from '@/i18n/routing';
 
 interface AuthContextType {
   user: User | null;
   organizations: Organization[];
   currentOrganization: Organization | null;
   isLoading: boolean;
-  isAuthReady: boolean; // ✅ NEW
+  isAuthReady: boolean;
   error: string | null;
   registerTrial: (
     email: string,
@@ -29,6 +30,7 @@ interface AuthContextType {
     organizationType: 'CLINIC' | 'LAB',
     planName?: string,
   ) => Promise<string>;
+  setUserLanguage: (language: string) => void;
   clearError: () => void;
 }
 
@@ -48,7 +50,13 @@ export function AuthProvider({ children }: { children: React.ReactNode }) {
     const organizations = payload?.organizations || [];
 
     if (payload?.user) {
-      return { user: payload.user as User, organizations };
+      return {
+        user: {
+          ...(payload.user as User),
+          language: (payload.user as User).language ?? 'en',
+        },
+        organizations,
+      };
     }
 
     if (payload?.id && payload?.email && payload?.name) {
@@ -57,6 +65,7 @@ export function AuthProvider({ children }: { children: React.ReactNode }) {
           id: payload.id,
           email: payload.email,
           name: payload.name,
+          language: payload.language ?? 'en',
         },
         organizations,
       };
@@ -272,6 +281,11 @@ export function AuthProvider({ children }: { children: React.ReactNode }) {
 
   const clearError = useCallback(() => setError(null), []);
 
+  const setUserLanguage = useCallback((language: string) => {
+    const normalized = isAppLocale(language) ? language : 'en';
+    setUser((current) => (current ? { ...current, language: normalized } : current));
+  }, []);
+
   const contextValue = useMemo(
     () => ({
       user,
@@ -285,6 +299,7 @@ export function AuthProvider({ children }: { children: React.ReactNode }) {
       logout,
       selectOrganization,
       createOrganization,
+      setUserLanguage,
       clearError,
     }),
     [
@@ -299,6 +314,7 @@ export function AuthProvider({ children }: { children: React.ReactNode }) {
       logout,
       selectOrganization,
       createOrganization,
+      setUserLanguage,
       clearError,
     ],
   );