Complete client UI i18n migration

Adds client-side i18n support, language settings, translated UI copy, and locale parity coverage.

packages/app/src/i18n/init.ts

@@ -0,0 +1,10 @@
+type InitReporter = (message: string, error: unknown) => void;
+
+export function observeI18nInit(
+  initPromise: Promise<unknown>,
+  report: InitReporter = console.error,
+): void {
+  initPromise.catch((error: unknown) => {
+    report("[i18n] Failed to initialize", error);
+  });
+}