Implement getInterfaceTextInLanguage and use API and Parser

Fallback for user language and content language is easiest to
resolve server-side. Also saves sending a lot of data to the
client that it doesn't need. Similar to how ResourceLoader only
sends 1 set of message values.

Bug: 50431
Bug: 52922
Change-Id: If8317ed6522a05d5a48a210ff43c97277b950a97
Merge "Implement getInterfaceTextInLanguage and use API and Parser"