Language fixes (#5384)

* Add some comments

* Decouple language settings

* Normalize on read/write

* Refactor

* Support device locale on app startup

* Cleanup, port to web

* Clean up comments

* Comment

* Try not to mutate

* Protect util handling, update test

* Dedupe array values

src/locale/helpers.ts

@@ -160,8 +160,13 @@ export function sanitizeAppLanguageSetting(appLanguage: string): AppLanguage {
   return AppLanguage.en
 }
 
+/**
+ * Handles legacy migration for Java devices.
+ *
+ * {@link https://github.com/bluesky-social/social-app/pull/4461}
+ * {@link https://xml.coverpages.org/iso639a.html}
+ */
 export function fixLegacyLanguageCode(code: string | null): string | null {
-  // handle some legacy code conversions, see https://xml.coverpages.org/iso639a.html
   if (code === 'in') {
     // indonesian
     return 'id'
@@ -176,3 +181,20 @@ export function fixLegacyLanguageCode(code: string | null): string | null {
   }
   return code
 }
+
+/**
+ * Find the first language supported by our translation infra. Values should be
+ * in order of preference, and match the values of {@link AppLanguage}.
+ *
+ * If no match, returns `en`.
+ */
+export function findSupportedAppLanguage(languageTags: (string | undefined)[]) {
+  const supported = new Set(Object.values(AppLanguage))
+  for (const tag of languageTags) {
+    if (!tag) continue
+    if (supported.has(tag as AppLanguage)) {
+      return tag
+    }
+  }
+  return AppLanguage.en
+}