Maybe it's just a bug that happens when language is undetermined and that's why it isn't even sent. It can't be Mastodon (I think) because otherwise we would see the message on Lemmy as sent but it wouldn't appear on Masto.
@Wander Lemmy may be getting a rejection when trying to post to the masto instance, since the original post with its list of replies doesn't live on Lemmy. I guess if you haven't you should try checking your logs for any errors?