It's working in articles but not on the main page. I'm happy to file a separate bug report if you like.
Steps to reproduce:
- Navigate to https://en.wikipedia.org/wiki/Main_Page while being logged-in and with ULS rewrite beta feature enabled
- Try clicking on the "language" button
- Nothing happens – except for the URL changing to https://en.wikipedia.org/wiki/Main_Page#/languages
- Repeating the same steps (or just opening the link https://en.wikipedia.org/wiki/Main_Page#/languages) with the beta feature disabled (or logged-out) works as expected.
- I confirmed the issue on dewiki's and frwiki's main page as well.
Additionally, I noticed we have issues even on the vector skin. On https://it.wikipedia.org/wiki/Pagina_principale, when I scroll down and click the languages button, the screen scrolls all the way up, hiding the language selector.