Sitemap links do not change according to the selected website language

By default, the COSCUP 2026 website language is set to `zh-Hant-TW`. When the language is changed to English, the links on the sitemap remain unchanged and lead to the `zh-Hant-TW` pages.

<img width="695" height="362" alt="Image" src="https://github.com/user-attachments/assets/10a9dd11-4428-42b2-8055-7356da7327b4" />