It could be that the community allowed languages isn’t set to allow “Undetermined” or you’re trying to post a comment with a language label that is not allowed. I’m not sure if there is a way to easily see as a user which language(s) was/were configured by the community mod(s).

That post seems broken, I can’t comment to it from Memmy.

I was able to comment to that post without a problem using Connect.

If the thread poster has a language set that you don’t have, even if they and you both post in English, you’ll get that error by not having “undetermined” selected as a language in your account settings. You can add it by logging into to the site you signed up on.

Testing my comment from Connect app.

Trying to comment here. If it does work, colour me confused 😂.