[web] fix language negotiation
Regression caused by commit 6fd0ac6506cb "[web-request] handle default
language earlier". We would handle language negotiation at request
init time, but overwrite the selected language with the site default
when calling set_cnx.
missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/share/pyshared/cubicweb/skeleton/debian/rules.tmpl)