@thomas I just figured it out and it's sillier than that: the languages listed in the "changed subscribed languages" menu are the ones used in some post _that has been loaded by the frontend_. I was only seeing English at first because all recent posts were in English; when I scrolled down far enough to see one in Finnish (necessitating the fetching of a couple more pages of history), Finnish suddenly appeared in the list.

I suppose I can live with that, but I would have done it differently.