i18n

src/lib/i18n/index.ts

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+import { setupI18n, type I18n, type Messages } from "@lingui/core";
+import { messages as enMessages } from "../../locales/en/messages.po";
+
+export const SUPPORTED_LOCALES = ["en", "es", "de", "ja"] as const;
+export type AppLocale = (typeof SUPPORTED_LOCALES)[number];
+export const DEFAULT_LOCALE: AppLocale = "en";
+
+// es is Mexican Spanish (translations + dayjs es-mx).
+export const LOCALE_LABELS: Record<AppLocale, string> = {
+  en: "English",
+  es: "Español",
+  de: "Deutsch",
+  ja: "日本語",
+};
+
+// Adding a language: add it to SUPPORTED_LOCALES + LOCALE_LABELS, add the
+// locale to lingui.config.ts, run `bunx lingui extract`, and translate the
+// generated src/locales/<locale>/messages.po.
+const catalogLoaders = import.meta.glob<{ messages: Messages }>(
+  "/src/locales/*/messages.po"
+);
+
+const messageCache: Partial<Record<AppLocale, Messages>> = {
+  [DEFAULT_LOCALE]: enMessages,
+};
+
+export function resolveLocale(candidate: string | null | undefined): AppLocale {
+  if (!candidate) return DEFAULT_LOCALE;
+  const base = candidate.toLowerCase().split("-")[0];
+  return (SUPPORTED_LOCALES as readonly string[]).includes(base)
+    ? (base as AppLocale)
+    : DEFAULT_LOCALE;
+}
+
+export async function ensureMessages(locale: AppLocale): Promise<Messages> {
+  const cached = messageCache[locale];
+  if (cached) return cached;
+
+  const loader = catalogLoaders[`/src/locales/${locale}/messages.po`];
+  if (!loader) return messageCache[DEFAULT_LOCALE]!;
+
+  const { messages } = await loader();
+  messageCache[locale] = messages;
+  return messages;
+}
+
+export function getCachedMessages(locale: AppLocale): Messages {
+  return messageCache[locale] ?? messageCache[DEFAULT_LOCALE]!;
+}
+
+export function hasCachedMessages(locale: AppLocale): boolean {
+  return messageCache[locale] !== undefined;
+}
+
+export function createI18n(locale: AppLocale): I18n {
+  const i18n = setupI18n();
+  i18n.load(locale, getCachedMessages(locale));
+  i18n.activate(locale);
+  return i18n;
+}
+
+export function parseAcceptLanguage(header: string | null): AppLocale {
+  if (!header) return DEFAULT_LOCALE;
+  for (const part of header.split(",")) {
+    const tag = part.split(";")[0]?.trim();
+    if (!tag) continue;
+    const base = tag.toLowerCase().split("-")[0];
+    if ((SUPPORTED_LOCALES as readonly string[]).includes(base)) {
+      return base as AppLocale;
+    }
+  }
+  return DEFAULT_LOCALE;
+}