Thanks @Moin for kindly linking to this thread, had missed it!
I second the need for this multi-language preference. Especially for logged-in users, but why not also for anonymous ones.
In Nordics and Europe, it’s so common for people to speak multiple languages, and currently the lack of this feature steers people to turn off the localisation. It’s not a bad thing per se, but would be nice to solve this when there’s more multilingual community and discussions.
I don’t have any ready solution on mind, but some thinking on top of @nat’s comments above: Could the language selector be more like checkbox settings, where you could select multiple languages you understand? There you also could mark (star) your main language, which would then be same as user.locale and affect the UI language?