Use system locale when Accept-Language header is not provided

Remove en_US as the default language when no header is provided, and use
None instead. Upon translation None will be defaulted to system as it
was before the translation changes.

Fixes bug 1214476

Change-Id: Ia18ae76f9dc3c93c7d14203af6f65b916500927e
Merge "Use system locale when Accept-Language header is not provided"