[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

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile