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: Ice7f55912a7bd6c727a7f8a2a1172871fe27a3dc
Merge "Use system locale when Accept-Language header is not provided"