[web-request] handle default language earlier We now read language negotiation header at initialisation time (previously done during ``set_session``). This simplify the set_session code for later reworking. There is no change in behavior, the user selected language continue to overwrite the one picked from the header/default when applicable. related to #2503918

"""cubicweb-%(cubename)s application package

%(longdesc)s
"""