diff --git a/client/src/api/client.ts b/client/src/api/client.ts
index 41f026b5..c8334e8a 100644
--- a/client/src/api/client.ts
+++ b/client/src/api/client.ts
@@ -380,7 +380,7 @@ export const weatherApi = {
 
 export const configApi = {
   getPublicConfig: (): Promise<{ defaultLanguage: string }> =>
-    fetch('/api/config').then(r => r.json()),
+    apiClient.get('/config').then(r => r.data),
 }
 
 export const settingsApi = {
diff --git a/client/src/i18n/TranslationContext.tsx b/client/src/i18n/TranslationContext.tsx
index e73e2caa..aee8ff71 100644
--- a/client/src/i18n/TranslationContext.tsx
+++ b/client/src/i18n/TranslationContext.tsx
@@ -54,19 +54,24 @@ export function isRtlLanguage(language: string): boolean {
 // Detects the user's preferred language from the browser/OS settings and maps
 // it to one of the supported language codes. Returns null if no match is found.
 export function detectBrowserLanguage(): string | null {
-  const browserLangs = navigator.languages?.length ? navigator.languages : [navigator.language]
+  if (typeof navigator === 'undefined') return null
+  const browserLangs = navigator.languages?.length
+    ? navigator.languages
+    : navigator.language ? [navigator.language] : []
   const supported = SUPPORTED_LANGUAGES.map(l => l.value)
 
   for (const lang of browserLangs) {
-    // Exact match (e.g. 'de', 'zh-TW')
-    if (supported.includes(lang)) return lang
+    // Exact match (e.g. 'de', 'zh-TW') — case-insensitive
+    const exactMatch = supported.find(s => s.toLowerCase() === lang.toLowerCase())
+    if (exactMatch) return exactMatch
 
     // Portuguese variants → our code is 'br' (pt-BR)
-    if (lang.startsWith('pt')) return 'br'
+    if (lang.toLowerCase().startsWith('pt')) return 'br'
 
-    // Prefix match (e.g. 'de-AT' → 'de', 'zh-CN' → 'zh')
-    const prefix = lang.split('-')[0]
-    if (supported.includes(prefix)) return prefix
+    // Prefix match (e.g. 'de-AT' → 'de', 'zh-CN' → 'zh') — case-insensitive
+    const prefix = lang.split('-')[0].toLowerCase()
+    const prefixMatch = supported.find(s => s.toLowerCase() === prefix)
+    if (prefixMatch) return prefixMatch
   }
 
   return null
diff --git a/client/src/pages/LoginPage.tsx b/client/src/pages/LoginPage.tsx
index 64b20666..8a309b62 100644
--- a/client/src/pages/LoginPage.tsx
+++ b/client/src/pages/LoginPage.tsx
@@ -2,7 +2,7 @@ import React, { useState, useEffect, useMemo, useRef } from 'react'
 import { useNavigate, useLocation } from 'react-router-dom'
 import { useAuthStore } from '../store/authStore'
 import { useSettingsStore } from '../store/settingsStore'
-import { SUPPORTED_LANGUAGES, useTranslation, detectBrowserLanguage } from '../i18n'
+import { SUPPORTED_LANGUAGES, useTranslation, detectBrowserLanguage } from '../i18n/TranslationContext'
 import { authApi, configApi } from '../api/client'
 import { getApiErrorMessage } from '../types'
 import { Plane, Eye, EyeOff, Mail, Lock, MapPin, Calendar, Package, User, Globe, Zap, Users, Wallet, Map, CheckSquare, BookMarked, FolderOpen, Route, Shield, KeyRound, ChevronDown } from 'lucide-react'
@@ -134,7 +134,7 @@ export default function LoginPage(): React.ReactElement {
 
     configApi.getPublicConfig()
       .then(({ defaultLanguage }) => { if (defaultLanguage) setLanguageTransient(defaultLanguage) })
-      .catch(() => {})
+      .catch((err) => console.warn('Failed to fetch default language config:', err))
   }, [setLanguageTransient])
 
   useEffect(() => {
diff --git a/client/src/store/settingsStore.ts b/client/src/store/settingsStore.ts
index 179527df..9e56e505 100644
--- a/client/src/store/settingsStore.ts
+++ b/client/src/store/settingsStore.ts
@@ -64,6 +64,8 @@ export const useSettingsStore = create<SettingsState>((set, get) => ({
   // Used for automatic detection (browser/server default) — only explicit user
   // choices via the UI should be persisted.
   setLanguageTransient: (lang: string) => {
+    const supported = ['de', 'en', 'es', 'fr', 'hu', 'nl', 'br', 'cs', 'pl', 'ru', 'zh', 'zh-TW', 'it', 'ar']
+    if (!supported.includes(lang)) return
     set((state) => ({ settings: { ...state.settings, language: lang } }))
   },
 
diff --git a/server/src/config.ts b/server/src/config.ts
index 1f3e787d..50a6dc97 100644
--- a/server/src/config.ts
+++ b/server/src/config.ts
@@ -102,4 +102,9 @@ export const ENCRYPTION_KEY = _encryptionKey;
 // DEFAULT_LANGUAGE sets the language shown on the login page before the user
 // selects one. Only applies when the user has no saved language preference.
 // Supported values: de, en, es, fr, hu, nl, br, cs, pl, ru, zh, zh-TW, it, ar
-export const DEFAULT_LANGUAGE = process.env.DEFAULT_LANGUAGE || 'en';
+const SUPPORTED_LANG_CODES = ['de', 'en', 'es', 'fr', 'hu', 'nl', 'br', 'cs', 'pl', 'ru', 'zh', 'zh-TW', 'it', 'ar'];
+const rawDefaultLang = process.env.DEFAULT_LANGUAGE || 'en';
+if (!SUPPORTED_LANG_CODES.includes(rawDefaultLang)) {
+  console.warn(`DEFAULT_LANGUAGE="${rawDefaultLang}" is not supported. Falling back to "en". Supported: ${SUPPORTED_LANG_CODES.join(', ')}`);
+}
+export const DEFAULT_LANGUAGE = SUPPORTED_LANG_CODES.includes(rawDefaultLang) ? rawDefaultLang : 'en';