Why is there no firstheading? Even on skins where nothing breaks as a result of it not being there, it's still a bit confusing and sometimes looks quite... odd.

I guess... is this a common thing? Like, is there some standard when extensions/components reasonably can/should be doing pages without a firstheading that we should be making sure we correctly support in general?

This seems to be a common thing within Translate. Many other pages like Special:SupportedLanguages, Special:TranslationStats etc., and even Special:Translate itself hide the firstHeading. I can’t recall any other extension doing so.

Special:AggregateGroups looks quote ugly on Vector as well, as the first <h2> is not full-width because of the help link:

This was just fixed (did not realize there was a task): https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Translate/+/644514

QAd on translatewiki.net.