This is probably more important for geographically-specific instances where the number of vernacular languages of the region served is constrained.
It would be a very nice thing to be able to filter the /public visible without being logged in to just the languages spoken in the region (for me, that’d be English, Spanish, a couple indigenous languages and maybe Vietnamese).
Likewise, the ability to set a default language filter on new profiles created would be handy (and could possibly match those of /public).
I feel that this could help geographic instances onboard new users without overwhelming them straight off with posts in languages the users of that instance are highly likely to be illiterate in. Presenting new users with a linguistic learning curve by default is not likely to be negotiated by casual users.