feat(i18n): detect initial locale from user language

apps/desktop/src/i18n/index.ts

@@ -17,18 +17,49 @@ const localeLoaders: Record<Exclude<Locale, "zh-CN">, () => Promise<{ default: L
   es: () => import("./locales/es"),
 };
 
-function normalizeLocale(value: string | null): Locale {
+export function normalizeLocale(value: string | null): Locale | null {
   if (value && supportedLocales.includes(value as Locale)) {
     return value as Locale;
   }
+  return null;
+}
+
+export function localeFromLanguageTag(value: string | null | undefined): Locale | null {
+  if (!value) return null;
+  const normalized = value.replace("_", "-").toLowerCase();
+  if (normalized === "zh-cn" || normalized.startsWith("zh-")) return "zh-CN";
+  if (normalized === "zh") return "zh-CN";
+  if (normalized === "en" || normalized.startsWith("en-")) return "en";
+  if (normalized === "es" || normalized.startsWith("es-")) return "es";
+  return null;
+}
+
+export function detectLocaleFromLanguages(languages: readonly string[]): Locale {
+  for (const language of languages) {
+    const locale = normalizeLocale(language) ?? localeFromLanguageTag(language);
+    if (locale) return locale;
+  }
   return defaultLocale;
 }
 
+function detectUserLocale(): Locale {
+  try {
+    const languages = globalThis.navigator?.languages;
+    const language = globalThis.navigator?.language;
+    const candidates = Array.isArray(languages) ? [...languages] : [];
+    if (language) candidates.push(language);
+    return detectLocaleFromLanguages(candidates);
+  } catch {
+    return defaultLocale;
+  }
+}
+
 const savedLocale = normalizeLocale(safeLocalStorageGet("dbx-locale"));
+const initialLocale = savedLocale ?? detectUserLocale();
 
 const i18n = createI18n({
   legacy: false,
-  locale: savedLocale,
+  locale: initialLocale,
   fallbackLocale: defaultLocale,
   messages: {
     "zh-CN": zhCN,
@@ -46,7 +77,7 @@ export async function loadLocaleMessages(locale: Locale) {
 }
 
 export async function loadSavedLocale() {
-  await loadLocaleMessages(savedLocale);
+  await loadLocaleMessages(initialLocale);
 }
 
 export async function setLocale(locale: Locale) {

packages/app-tests/i18nLocale.test.ts

@@ -0,0 +1,29 @@
+import { strict as assert } from "node:assert";
+import test from "node:test";
+import {
+  detectLocaleFromLanguages,
+  localeFromLanguageTag,
+  normalizeLocale,
+} from "../../apps/desktop/src/i18n/index.ts";
+
+test("normalizes exact supported locales", () => {
+  assert.equal(normalizeLocale("en"), "en");
+  assert.equal(normalizeLocale("es"), "es");
+  assert.equal(normalizeLocale("zh-CN"), "zh-CN");
+  assert.equal(normalizeLocale("fr-FR"), null);
+  assert.equal(normalizeLocale(null), null);
+});
+
+test("maps user language tags to supported locales", () => {
+  assert.equal(localeFromLanguageTag("zh-Hans-CN"), "zh-CN");
+  assert.equal(localeFromLanguageTag("zh_TW"), "zh-CN");
+  assert.equal(localeFromLanguageTag("en-US"), "en");
+  assert.equal(localeFromLanguageTag("es-MX"), "es");
+  assert.equal(localeFromLanguageTag("fr-FR"), null);
+});
+
+test("detects the first supported user locale and falls back to Chinese", () => {
+  assert.equal(detectLocaleFromLanguages(["fr-FR", "es-MX", "en-US"]), "es");
+  assert.equal(detectLocaleFromLanguages(["de-DE", "ja-JP"]), "zh-CN");
+  assert.equal(detectLocaleFromLanguages([]), "zh-CN");
+});