Add Tests for Localization

[CarliJoy]

Currently: https://github.com/CarliJoy/django-pint/blob/75e516a819a16422b93bad873e064a3ce179a927/src/quantityfield/fields.py#L206

is not covered in tests.

It would be nice to look in django test to figure out some tests that would test the localization of the field.

@cornelv also this would be an Issue worth spending time one