I’m working on a feature that will set the locale for new users based off the accept-language header that is sent by their browser. It should do some of what you are looking for here. It will hopefully make sure that new users on multilingual forums get signed up with the correct locale. Users on multilingual forums will get their emails (including the activation email) in the correct language. This should be ready right away.
I would like to add a locale-switcher to the header for the case of a non-logged-in user whose locale is different than the site’s default locale. This is to deal with the possibility that their locale has been set incorrectly. If this gets the go ahead, it would be easy to add the locale-switcher to the header under other conditions, either through a site setting or a plugin.