fix(settings): keep the language and save settings one after the other

src/components/AppHeader.jsx

@@ -71,7 +71,16 @@ export default function AppHeader({isAuthenticated, onLogout}) {
         toast.error(t('NAV.VIEW_AS.ERROR'));
     };
 
-    const toggleLanguage = () => setLanguage(i18n.language === 'de' ? 'en' : 'de');
+    // Unsaved, the next login would switch back to Auth's language.
+    const toggleLanguage = async () => {
+        try {
+            const response = await setLanguage(i18n.language === 'de' ? 'en' : 'de');
+            if (response && !response.ok) toast.error(t('SETTINGS.ERROR_SAVE'));
+        } catch (err) {
+            console.error('Language save error:', err);
+            toast.error(t('INFO.ERROR.NETWORK_FAILURE'));
+        }
+    };
     const toggleTheme = () => setDarkMode(!isDarkMode);
     const ThemeIcon = isDarkMode ? Sun : Moon;