Arend Spijker worked on refining user interface terminology in the lichess-org/lila repository, focusing on improving clarity and localization readiness. He updated the UI to rename 'castling' and 'en passant' to 'castling rights' and 'en passant rights,' respectively, ensuring terminology accurately reflected chess rules and supported internationalization workflows. Using TypeScript and XML, Arend applied UI/UX design principles to minimize user confusion and maintain consistency across languages. The work was delivered as a targeted, traceable feature with a clear commit history, demonstrating careful scope management. While the contribution was focused, it addressed a nuanced aspect of user experience and localization.