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frontend/src/i18n.ts

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+import i18n from "i18next";
+import { initReactI18next } from "react-i18next";
+
+import en from "./locales/en/common.json";
+import de from "./locales/de/common.json";
+
+const LANGUAGE_STORAGE_KEY = "contract-companion.language";
+const SUPPORTED_LANGUAGES = ["en", "de"] as const;
+
+type SupportedLanguage = (typeof SUPPORTED_LANGUAGES)[number];
+
+function normalizeLanguage(value?: string | null): SupportedLanguage {
+  if (!value) return "en";
+  const lower = value.toLowerCase();
+  const match = SUPPORTED_LANGUAGES.find((lang) => lower.startsWith(lang));
+  return match ?? "en";
+}
+
+function detectInitialLanguage(): SupportedLanguage {
+  if (typeof window === "undefined") {
+    return "en";
+  }
+
+  try {
+    const stored = window.localStorage.getItem(LANGUAGE_STORAGE_KEY);
+    if (stored) {
+      return normalizeLanguage(stored);
+    }
+  } catch {
+    // Access to localStorage might be blocked (e.g., privacy mode); ignore and fall back to navigator.
+  }
+
+  return normalizeLanguage(window.navigator.language);
+}
+
+const initialLanguage = detectInitialLanguage();
+
+i18n.use(initReactI18next).init({
+  resources: {
+    en: { translation: en },
+    de: { translation: de }
+  },
+  lng: initialLanguage,
+  fallbackLng: "en",
+  supportedLngs: ["en", "de"],
+  interpolation: {
+    escapeValue: false
+  }
+});
+
+if (typeof window !== "undefined") {
+  const applyLanguageSideEffects = (lang: string) => {
+    const normalized = normalizeLanguage(lang);
+    try {
+      window.localStorage.setItem(LANGUAGE_STORAGE_KEY, normalized);
+    } catch {
+      // Ignore storage write failures (private mode, etc.)
+    }
+    if (typeof document !== "undefined") {
+      document.documentElement.setAttribute("lang", normalized);
+    }
+  };
+
+  applyLanguageSideEffects(initialLanguage);
+  i18n.on("languageChanged", applyLanguageSideEffects);
+}
+
+export default i18n;