Worked on the zitadel/zitadel repository to enhance localization and login user experience by implementing Login UI Locale Synchronization using TypeScript. Developed a feature that reads ui_locales from OIDC login requests, validates them, and sets the application’s language cookie based on the first valid locale, ensuring the login UI accurately reflects the user’s language preference. Addressed a regression where locale settings were previously ignored in the v2 login UI, restoring expected behavior. Added comprehensive test coverage to verify locale handling and prevent future regressions. This work improved localization consistency and accessibility across the login and application interfaces, supporting multilingual user needs.