https://vote.wikimedia.org/wiki/Special:SecurePoll/vote/457 has some sort of translation system, though quite broken (T97922) and uselang works, but I can't select interface language due to T58464 (applies to everyone because it's not a CentralAuth wiki). The manual language selector for 18 languages is clunky and unfair for the 330+ MediaWiki languages not listed.
- set $wgULSAnonCanChangeLanguage = true (probably requires disabling the cache, or perhaps bypassing varnish entirely, but little else; works perfectly on normal wikis and this is an extremely low traffic wiki);
- connect the wiki to CentralAuth and allow users to login (partial solution because autologin often fails);
- if all else fails, add the language selector used by Wikidata and Commons to the wiki's Common.js: it works correctly for most languages, though it often needs a click etc.