As discussed in
XE-552, Thomas Mortagne and I agreed that interface language and page content language are two unrelated dimensions.
With the new Colibri skin in XE2.1, I think we are in a good position to seperate them.
1. Move the content language switcher from the site title bar to the page title bar.
2. In register page and personal profile page, add a "preferred language" setting. It's the interface language as well as the default content language.