It would be useful to have an ordered list of user languages in preferences, rather than just one.
This would particularly help on wikis with more than one content language, such as Wikidata or Commons. Pages, watchlists, recent etc. could optionally show all the users' languages, while hiding ones the user didn't select.
For the interface itself, it could use the first language then fallback through the others in some order (this is a bit complicated since the languages themselves have fallbacks).