See https://github.com/mozilla/zamboni/blob/master/apps/translations/query.py#L85

When fetching translations for a model to build the translated django instance, we only care about the current language and the fallback language (default_language/default_locale usually) for the model. So if you don't have a Translation in one of these languages, well, you get an empty name/description/etc.

See https://bugzilla.mozilla.org/show_bug.cgi?id=915652 for more info. Currently, Collections API avoids this because we return all the translations every time and we don't really care about what the django instance has.