fix: default setup language to system locale (#500)

shared/language.ts

@@ -0,0 +1,24 @@
+export const SUPPORTED_LANGUAGE_CODES = ['en', 'zh', 'ja'] as const;
+
+export type LanguageCode = (typeof SUPPORTED_LANGUAGE_CODES)[number];
+
+const SUPPORTED_LANGUAGE_CODE_SET = new Set<string>(SUPPORTED_LANGUAGE_CODES);
+
+function normalizeLocale(locale: string | null | undefined): string {
+  return locale?.trim().toLowerCase().replaceAll('_', '-') ?? '';
+}
+
+export function resolveSupportedLanguage(
+  locale: string | null | undefined,
+  fallback: LanguageCode = 'en',
+): LanguageCode {
+  const normalizedLocale = normalizeLocale(locale);
+  if (!normalizedLocale) {
+    return fallback;
+  }
+
+  const [baseLanguage] = normalizedLocale.split('-');
+  return SUPPORTED_LANGUAGE_CODE_SET.has(baseLanguage)
+    ? (baseLanguage as LanguageCode)
+    : fallback;
+}