That is already possible. There is a configuration option to “allow users to choose their own language interface preference”.
However, it does not solve the issue described in this feature request. If I am an admin of my board, my preference will be global for me which I don’t want. I want to be able to access admin interface in English while the public facing area and inline moderation tools all in a different language of my choice. For now, it would require me to change the language each time I switch back and forth between the two interfaces. Unless I chose to use two separate browsers with no shared sessions.