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: I0fe22c526710e69ae0731e7d0b42170e6f3a8523
Merge "Use system locale when Accept-Language header is not provided"