Ok, something seems to be very wrong here. I did this

22:06 . . Admin (talk | contribs | block) limited languages for translatable page Abc to تونسي and Acèh ‎

and then this

22:17 . . Admin (talk | contribs | block) discouraged translation of C/B ‎

and now on the translation interface for C/B it shows

Translation of the wiki page Abc from English (en).

Translating to English is not a priority for this page.

A translation administrator decided to focus the translation efforts on تونسي, Acèh.

Note that it says "Abc" wiki page instead of C/B.

Looks like that error is only appearing when you select English from the translation interface.

Thanks for testing. https://www.mediawiki.org/wiki/Help:Extension:Translate/Page_translation_administration says «hide them from the translation interface. [...] Discouraging and its reversion are done from Special:PageTranslation.»

Works for me with Meta's discouraged pages:

Oh thanks. After reading the docs, it looks like what I thought about this feature was wrong. I expected the translation interface to show a message at the top saying it's discouraged (though it probably should). Discouraging doesn't prevent new translations; it only hides them from message group selectors (and this works properly).

But the wrong message being shown for an unrelated group still exists.

But the wrong message being shown for an unrelated group still exists.

I'm not sure I understood this part. Does "Direct link to Special:Translate for disabled group shows mentions of another group" describe what you observe?

Looks like state.group is undefined when source language is selected because TuxMessageTable is never loaded (and this has the information needed by the JS to set state.group correctly). This is also the root cause for Special:Translate always showing "Translation of the wiki page 100wikidays from English (en)." for translatable pages when English is selected. See https://meta.wikimedia.org/w/index.php?title=Special:Translate&group=page-IP+block+exempt&action=page&filter=&language=en for example.

Change 424607 had a related patch set uploaded (by Nikerabbit; owner: Nikerabbit):
[mediawiki/extensions/Translate@master] Move language validation from Special:Translate to ApiQueryMessageCollection

https://gerrit.wikimedia.org/r/424607

Change 424607 merged by jenkins-bot:
[mediawiki/extensions/Translate@master] Move language validation from Special:Translate to ApiQueryMessageCollection

https://gerrit.wikimedia.org/r/424607