I believe this has to do with Connect not having a language selection dropdown when commenting. If I make the same comment that gives the error on Connect through a web browser, the comment will not post unless I select a language.
This is typically caused by not having "undetermined" set as a language. If the thread poster has a language set that you don't have, even if they post in English and you do the same, when you try to reply you get this error. This is a Lemmy issue rather than Connect specific. You'll need to login to your account on the site you registered on, go into Settings, and have "undetermined" as a selected language.
It never occurred to me that it might be a filter for bad language. Is that a thing? I'm skeptical. I think you're right that it's just a bug. Don't know if it's Connect or Lemmy that has the bug though.
I thought it was just a “you can’t call that dude a cunt” feature that subs can opt in our out of and they were just making sure I wasn’t offending anyone.