Yea unfortunately this seems like it might be the only solution to the problem. It's such a weird issue because the logic we use seemingly makes perfect sense but I guess it doesn't to Lemmy.
We try to use the language of the post or comment you are responding to (i.e. if the language of the post is English, the reply will use the same language). But then there are cases where the language is set to undefined and Lemmy feels fine letting the post be made with the undefined language, but it won't let you make a comment with undefined.