[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
include *.py
include */*.py
recursive-include data *.gif *.png *.ico *.css *.js
recursive-include i18n *.po
recursive-include wdoc *