Adding modeltranslation languages does not work with Default values

[LePetitTim]

Model translation does not allow to add a new language whenever we want to add a new one.
Configuration for languages is static and should not be updated.
When we want to add a new language, we use the command sync_translation_fields which is used normally to add translations on new fields.
This command is not made for our use case.
If there is a default value (for example a boolean field), the command does not work on this field.

Solution :

Add default values management in settings (we can change the behavior for each users)
Overload the sync_translation_fields for adding default values management:
https://github.com/deschler/django-modeltranslation/blob/0735755070df9e96c2e745a463518fda7f02e3c7/modeltranslation/management/commands/sync_translation_fields.py#L145