I'm using Jerboa and while I was able to comment initially, it first started saying "deleted" immediately after posting and now it says "language_not_allowed" and the comment is gone, irrespective of content. What's up with that?
It happens if you are replying to someone from an instance with strong language requirements (e.g. Kbin). You have to switch to the web interface to be able to select a language.
I just checked this with a comment from a kbin user I previously couldn't reply to and it worked immediately. Just had to select English language rather than leaving it blank.