[web] Simplify a bit language handling Drop `set_default_language` and `set_user_language` by considering that: * before being bound to the user, request will have the site's default language * cnx has the user's preferred language, hence get it back to the request when it's bound to a connection The first point requires some change to cubicweb's vreg faking so it doesn't break. That should be enough.

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules)