[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.
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/