API calls to add files to translation jobs don't apply language code mapping

[bcdickinson]

In particular, this line get's the unmapped codes from the wagtail-localize models, which causes validation errors if there are mismatches between the Django config and Smartling: https://github.com/mozilla/wagtail-localize-smartling/blob/4e5df47fb91d137aef85e0e2f12104ed543144ff/src/wagtail_localize_smartling/api/client.py#L379