I believe it happens if the OP sets the language to something other than undefined when creating their post. It is an option I noticed when using lemmy.world directly, I set it to English on a post I made, someone PM’d me and said they couldn’t leave a comment, I changed the post’s language to undefined via an edit, and then the person was able to comment on it. I was able to recreate the issue on my end with both an app and also with the lemmy.world browser UI with language set to undefined.

I think being able to mark certain posts as certain languages has its advantages, but when most lemmy front ends don’t support language settings, it just causes silent errors and confusion.