A solution has been found. Previously I was clicking hamburger, admin, users, USERNAME, and it took me to /admin/users/INDEX/USERNAME, where I saw a list of many settings for the user, but nothing about their preferred language.
But instead, I’m apparently supposed to click onward from that page on show public profile, preferences, interface, and it takes me to /u/USERNAME/preferences/interface, and there I can set the user’s interface language.
Thank you.