TIL that you can't respond on a comment tree if the top-level comment has set a language unless you set the same language
If you don't choose the same language as the top-level comment you cannot answer in the same tree. The wheel will just keep spinning but when you look at the API request you see that it returns something like "bad language" and upon choosing the right language you can comment again!
I think the original intention was that communities exists where people speaking different languages coexist and only see content they actively understand.
The reality now of course looks like every language group will have a dedicated community, often even on a country-specific(-ish) instance, rendering the whole idea obsolete.