Fix AttributeError for "lang" on repo/client connections
This is a regression introduced by b48020a80dc3, which removed call to
set_language from the `req._set_user` method. This is fine, but we still want a
language on connections, so we've to handle it if necesary in
Connection.__enter__ (i.e. once connection is properly open).
When using pyramid front-end, which has a users cache, we've to cache its
language as well because we must not access its preferred_language method since
it's not bound to a proper connection.