On Wikidata my userpage (https://www.wikidata.org/wiki/User:Multichill) contains {{#babel:nl|en-3|de-1|fr-1|es-0|pt-0}} so when I edit an item or a property I see the interface in English, German, Spanish, French, Dutch & Portuguese. I noticed that https://www.wikidata.org/wiki/User:Coyau got deleted. On meta that page contains {{#babel:fr-N|en-1}}, but the local interface on Wikidata won't show up in French and English for this user.
That #babel on meta should have the same effect on the Wikidata interface as a local #babel.

The bug T90398 relates to this: The babel extension uses categories to store it's information. So for example if you look at my user page you'll see "Categories:User nlUser nl-NUser enUser en-3User deUser de-1User frUser fr-1User esUser es-0User ptUser pt-0". Having that bug solved would probably cover this story, solving it in a different cleaner way would be nice to (maybe even better).