Fix translate static messages in response

Messages created statically (during import) were not being
translated in responses when the Accept-Language header was
used to set the expected language in the response. The static
messages were being created before the _ built-in had been
installed by gettextutils.install().

Change-Id: Ie56b1d3a836bc5f2262d7af68f803a08ebdf016f
Resolves-Bug: #1215192
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 "Fix translate static messages in response"
Merge "Use system locale when Accept-Language header is not provided"
Fixes a link in the documentation

Change-Id: I4872cd24511df995b023a47edad3176a54a49504