Because this code is hit before that fall-back.

The issue is in Django's inconsistent support of sub-languages (en-us); sometimes the fall-back is "en", sometimes it's "en-us".  

That's wrong, wrong, wrong.  The code needs to be consistent.  I believe there are two bugs in Django (one with the interaction between caching and internationalization modules, the other in handling settings.LANGUAGE_CODE differently than other language settings)