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!
As someone with less tolerance for native speakers failing at doing so, I wonder whether this can be repurposed.
But maybe a regex filter on such like /(my|the|some) emails/ or /literally/ would be the same. As that’s possible as a client-side feature, it doesn’t even need to be in on the server side until it’s a federation filter.