Gotta love UI design..

So, if I select all languages, not just undetermined, I can finally see and respond to comments.

It means that if a new language is added, nobody will see it, which is clearly user antagonistic.

The upshot is that apart from DM between Mastodon and Lemmy and back, everything appears to be working.

Thank you for your perseverance.

As for the spam, seems to me that blocking that instance explicitly would resolve the issue.