Always return all translations in Collections API, and add default_language (bug 907181)

https://bugzilla.mozilla.org/show_bug.cgi?id=907181

With that change, we now accept and return dictionaries for translated fields. In addition, there is a new default_language field to match what we are doing with Translation objects in other models.

There is a lot of confusion between locale vs language in various places in our code, I tried to avoid that confusion and use "language" everywhere here, because that's what made sense to me and because that's what Translation objects use internally, even if sometimes the doc says otherwise.