Checklist

• [x I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

Make a model in django that has a unique field, make a viewset with default model serializer for it, create an instance of the model with api. Try to create another instance with the same value for unique field as first instance while your language is set to something other than en, it doesn't translate the model name. The error message is %(model_name)s with this %(field_labels)s already exists. and the model_name doesn't get translated.

Expected behavior

It to translate the whole error message.

Actual behavior

It doesn't.

I tried to fix it in this pull request: #4299 but after seeing how many tests have failed I concluded that you're using method get_field_kwargs in field_mapping.py for other purposes than making error messages. So I don't know how to fix it that doesn't break other parts.