From the docs: https://docs.djangoproject.com/en/2.0/to...anslation/
Also see: https://djangobook.com/internationalization/
Quote:If a base language is available but the sublanguage specified is not, Django uses the base language. For example, if a user specifies de-at (Austrian German) but Django only has de available, Django uses de.
Also see: https://djangobook.com/internationalization/