Ensure the `language_code` is proper on form construction
Ensure the `language_code` is proper on form construction

When the `language_code` isn't part of the `LANGUAGES` setting,
raise an error beforehand. It's pointless to check for form errors as
the setting is likely provided indirectly.

django-parler/django-parler,django-parler/django-parler,edoburu/django-parler,edoburu/django-parler