Make sure the default_language, if any, is first

[jnm]

Closes #1942. Marking as high priority since we accidentally deployed a half-baked version of #1913 and need this to finish the job properly.

[noliveleger]

@jnm, I could not directly comment on these lines in asset.py

L297    def _prioritize_translation(self, content, translation_name, is_new=False):
L298        _translations = content.get('translations')
L299        _translated = content.get('translated', [])
L300        if is_new and (translation_name in _translations):
L301            raise ValueError('cannot add existing translation')
L302        elif (not is_new) and (translation_name not in _translations):

I don't remember if content always has a translations key but in case it does not, L300 and L302 should crash because of None iteration.
Like L299, I would set a default value (empty list or dict) to content.get('translations') on like L298

[jnm]

@noliveleger, will assert that the content has translations per our discussion.

[jnm]

@noliveleger, what do you think about this approach?