Allow language selection to be user specific with Multiuser

from what I understand, changing the interface language when using the Multiuser plugin is disallowed by default as hanging the language would change it for everybody.

Ideally this would be a per-user setting, stored in the user's browser, as currently we have some Russian and English proxies, which people in other countries aren't able to use due to the language being stuck as the region default.

Making it per-user and allowing it to be changed by default with the Multiuser proxy would solve this problem.