add language preference to user profile

[fsmanuel]

Now that translations are landing, it would be useful that users can choose their own language. Also on signing up for a new account, it would be great if a language chooser would be visible, which would save it as preference for the new user as well.
Also the site language should be configured somewhere sensible in config/ (instead of hardcoded in application_controller).
Branch foodcoop contains some little bits already.