[1.1.X] Fixed #7720 - Fallback to the base language if the sub language given...

[1.1.X] Fixed #7720 - Fallback to the base language if the sub language given in the language cookie doesn't exist. Thanks, djoume and Ramiro Morales.

Backport of r12442.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12452 bcc190cf-cafb-0310-a4f2-bffc1f526a37