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!
Why is that feature even a thing then?
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.
Hopefully they just focus on implementing a convenient way to translate comments so people who speak different languages can easily communicate.