Fixed #6409 -- Unbreak compound locale name parsing (e.g. zh-cn).

This was inadvertently broken back in [6608]. Slightly backwards-incompatible:
people specifying "es_AR" in their LANGUAGES list will need to change that to
"es-ar". Thanks, simonb and Ramiro Morales for making the effort to fix this.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7091 bcc190cf-cafb-0310-a4f2-bffc1f526a37