[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/bin/cubicweb-ctl usr/bin/
usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl