[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.

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cubicweb cubicweb-common