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).
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.