feat(i18n): foundation — react-i18next, language switcher, server-persisted choice

Set up react-i18next with locale files auto-loaded per area (Vite glob), a compact
LanguageSwitcher, and language as a server-persisted preference (preferences.language)
mirrored to localStorage. On first login the default UI language is guessed from the
Google-reported locale (hu/en/de, else English). vite-env.d.ts types the build-time env.

frontend/src/App.tsx

@@ -1,6 +1,8 @@
 import { useEffect, useState } from "react";
+import { useTranslation } from "react-i18next";
 import { useQuery } from "@tanstack/react-query";
 import { api, HttpError, type FeedFilters } from "./lib/api";
+import { setLanguage, isSupported, type LangCode } from "./i18n";
 import {
   applyTheme,
   DEFAULT_THEME,
@@ -61,6 +63,7 @@ function loadInitialFilters(): FeedFilters {
 }
 
 export default function App() {
+  const { t } = useTranslation();
   const [theme, setThemeState] = useState<ThemePrefs>(() => loadLocalTheme());
   const [filters, setFiltersState] = useState<FeedFilters>(loadInitialFilters);
   const [view, setView] = useState<"grid" | "list">("grid");
@@ -124,6 +127,7 @@ export default function App() {
       setSidebarLayoutState(l);
       saveLayoutLocal(l);
     }
+    if (isSupported(prefs.language)) setLanguage(prefs.language);
     if (prefs.notifications) configureNotifications(prefs.notifications);
     if (typeof prefs.hints === "boolean") setHintsEnabled(prefs.hints);
     if (typeof prefs.performanceMode === "boolean")
@@ -133,8 +137,8 @@ export default function App() {
     if (meQuery.data?.role === "admin" && pending > 0) {
       notify({
         level: "warning",
-        title: "Access requests",
-        message: `${pending} pending — review in Settings → Account.`,
+        title: t("common.accessRequestsTitle"),
+        message: t("common.accessRequestsMessage", { count: pending }),
       });
     }
     // eslint-disable-next-line react-hooks/exhaustive-deps
@@ -149,15 +153,21 @@ export default function App() {
     setView(v);
     api.savePrefs({ view: v }).catch(() => {});
   }
+  function changeLanguage(code: LangCode) {
+    setLanguage(code);
+    api.savePrefs({ language: code }).catch(() => {});
+  }
 
   if (meQuery.isLoading)
-    return <div className="min-h-screen grid place-items-center text-muted">Loading…</div>;
+    return (
+      <div className="min-h-screen grid place-items-center text-muted">{t("common.loading")}</div>
+    );
   if (meQuery.error instanceof HttpError && meQuery.error.status === 401)
     return <Login />;
   if (meQuery.error)
     return (
       <div className="min-h-screen grid place-items-center text-muted">
-        Something went wrong.
+        {t("common.somethingWrong")}
       </div>
     );
 
@@ -171,6 +181,7 @@ export default function App() {
         setPage={setPage}
         onOpenSettings={() => setSettingsOpen(true)}
         onOpenAbout={() => setAboutOpen(true)}
+        onChangeLanguage={changeLanguage}
         onGoToFullHistory={() => {
           setChannelFilter("needs_full");
           setPage("channels");

frontend/src/components/LanguageSwitcher.tsx

@@ -0,0 +1,62 @@
+import { useRef, useState } from "react";
+import { Check, Globe } from "lucide-react";
+import { LANGUAGES, type LangCode } from "../i18n";
+
+// Compact language picker (globe + current code). Presentational: the parent decides what
+// changing the language does (set it; persist server-side when signed in).
+export default function LanguageSwitcher({
+  value,
+  onChange,
+  align = "right",
+}: {
+  value: LangCode;
+  onChange: (code: LangCode) => void;
+  align?: "left" | "right";
+}) {
+  const [open, setOpen] = useState(false);
+  const closeTimer = useRef<ReturnType<typeof setTimeout> | null>(null);
+  const current = LANGUAGES.find((l) => l.code === value) ?? LANGUAGES[0];
+
+  function openNow() {
+    if (closeTimer.current) clearTimeout(closeTimer.current);
+    setOpen(true);
+  }
+  function closeSoon() {
+    closeTimer.current = setTimeout(() => setOpen(false), 200);
+  }
+
+  return (
+    <div className="relative" onMouseEnter={openNow} onMouseLeave={closeSoon}>
+      <button
+        onClick={() => setOpen((o) => !o)}
+        title={current.label}
+        className="flex items-center gap-1.5 px-2.5 py-1.5 rounded-lg text-sm text-muted hover:text-fg hover:bg-card transition"
+      >
+        <Globe className="w-4 h-4" />
+        <span className="text-xs font-semibold uppercase">{current.code}</span>
+      </button>
+
+      {open && (
+        <div
+          className={`glass absolute ${
+            align === "right" ? "right-0" : "left-0"
+          } mt-1 w-40 rounded-xl p-1.5 z-40 animate-[popIn_0.16s_ease]`}
+        >
+          {LANGUAGES.map((l) => (
+            <button
+              key={l.code}
+              onClick={() => {
+                onChange(l.code);
+                setOpen(false);
+              }}
+              className="w-full flex items-center justify-between gap-2 text-sm px-2 py-2 rounded-lg text-muted hover:text-fg hover:bg-card transition"
+            >
+              <span>{l.label}</span>
+              {l.code === value && <Check className="w-4 h-4 text-accent" />}
+            </button>
+          ))}
+        </div>
+      )}
+    </div>
+  );
+}

frontend/src/i18n/index.ts

@@ -0,0 +1,58 @@
+import i18n from "i18next";
+import { initReactI18next } from "react-i18next";
+
+// Endonyms (shown as-is regardless of the active language — the convention for language pickers).
+export const LANGUAGES = [
+  { code: "en", label: "English" },
+  { code: "hu", label: "Magyar" },
+  { code: "de", label: "Deutsch" },
+] as const;
+export type LangCode = (typeof LANGUAGES)[number]["code"];
+const SUPPORTED = LANGUAGES.map((l) => l.code) as readonly string[];
+export const LANG_KEY = "siftlode.lang";
+
+// Auto-load every locale file (locales/<lang>/<area>.json) and merge each <area> under one
+// `translation` namespace, so components use t("area.key") and adding an area needs no wiring.
+const mods = import.meta.glob("./locales/*/*.json", { eager: true }) as Record<
+  string,
+  { default: Record<string, unknown> }
+>;
+
+const resources: Record<string, { translation: Record<string, unknown> }> = {};
+for (const [path, mod] of Object.entries(mods)) {
+  const m = path.match(/\/locales\/([a-z]{2})\/([a-zA-Z0-9_]+)\.json$/);
+  if (!m) continue;
+  const [, lang, area] = m;
+  (resources[lang] ??= { translation: {} }).translation[area] = mod.default;
+}
+
+export function isSupported(code: string | null | undefined): code is LangCode {
+  return !!code && SUPPORTED.includes(code);
+}
+
+// Initial language before the server preference is known: stored choice, else the browser
+// language, else English. (After login, App adopts the server-persisted preference.)
+export function detectInitialLang(): LangCode {
+  const stored = localStorage.getItem(LANG_KEY);
+  if (isSupported(stored)) return stored;
+  const nav = (navigator.language || "en").slice(0, 2).toLowerCase();
+  return isSupported(nav) ? nav : "en";
+}
+
+const initial = detectInitialLang();
+void i18n.use(initReactI18next).init({
+  resources,
+  lng: initial,
+  fallbackLng: "en",
+  interpolation: { escapeValue: false },
+  returnNull: false,
+});
+document.documentElement.lang = initial;
+
+export function setLanguage(code: LangCode): void {
+  void i18n.changeLanguage(code);
+  localStorage.setItem(LANG_KEY, code);
+  document.documentElement.lang = code;
+}
+
+export default i18n;

frontend/src/lib/version.ts

@@ -1,10 +1,8 @@
 // Build info baked into the SPA at build time (Vite inlines VITE_*-prefixed env).
 // Injected via Docker build-args (see Dockerfile); falls back to "dev" for un-stamped builds.
-const env = (import.meta as unknown as { env: Record<string, string | undefined> }).env;
-
-export const FRONTEND_VERSION = env.VITE_APP_VERSION || "dev";
-export const FRONTEND_SHA = env.VITE_GIT_SHA || "unknown";
-export const FRONTEND_BUILD_DATE = env.VITE_BUILD_DATE || "";
+export const FRONTEND_VERSION = import.meta.env.VITE_APP_VERSION || "dev";
+export const FRONTEND_SHA = import.meta.env.VITE_GIT_SHA || "unknown";
+export const FRONTEND_BUILD_DATE = import.meta.env.VITE_BUILD_DATE || "";
 
 // Key for the "last version the user has seen release notes for" (drives the banner).
 export const SEEN_VERSION_KEY = "siftlode.seenVersion";

frontend/src/main.tsx

@@ -5,6 +5,7 @@ import App from "./App";
 import ErrorBoundary from "./components/ErrorBoundary";
 import PrivacyPolicy from "./components/legal/PrivacyPolicy";
 import Terms from "./components/legal/Terms";
+import "./i18n";
 import "./index.css";
 
 const queryClient = new QueryClient({

frontend/src/vite-env.d.ts

@@ -0,0 +1,7 @@
+/// <reference types="vite/client" />
+
+interface ImportMetaEnv {
+  readonly VITE_APP_VERSION?: string;
+  readonly VITE_GIT_SHA?: string;
+  readonly VITE_BUILD_DATE?: string;
+}