Now it happened the second time that I posted with the wrong language setting. I see a little i next to the comment, but I cannot change it anymore. I also cannot see and copy the comment to post it with the correct language. Could Summit perhaps warn me that I have a not-acceptable language set? Or let me change the setting after the error occurs?

I just tested this on your instance (feddit.org) and I was able to change my language after sending the comment. Are you seeing an error if you change your language and try to update your comment?

I forgot to mention that it was in /c/europe, sorry. Apparently only English is allowed there, but my default language was German. I can still find the post from the failed user actions. The problem is that I cannot interact with the failed post at all - with the exception of the circled (i) at the top, which leads to the error message (and now I see that I also could long-press the text and copy it). The row of buttons below the text (upvote/downvote, comment, bookmark, three-dot-menu) is missing.

Edit: now I copied the text and posted it again (this time making sure that it is set as English), and it worked.

Oh I see. That is an interesting problem. I can look into if there is a nicer solution.

Happens to me too