refactor: extract language utilities to lang.ts and adjust CI pipeline triggers.

packages/next-utils/src/lang.ts

@@ -0,0 +1,6 @@
+export const languages = ["en", "de"] as const;
+export type Lang = (typeof languages)[number];
+
+export function isValidLang(lang: string): lang is Lang {
+  return (languages as readonly string[]).includes(lang);
+}