Переопределить в модели метод unique_error_message, родной выглядит так, дальше, надеюсь, справитесь
def unique_error_message(self, model_class, unique_check):
opts = model_class._meta
params = {
'model': self,
'model_class': model_class,
'model_name': capfirst(opts.verbose_name),
'unique_check': unique_check,
}
# A unique field
if len(unique_check) == 1:
field = opts.get_field(unique_check[0])
params['field_label'] = capfirst(field.verbose_name)
return ValidationError(
message=field.error_messages['unique'],
code='unique',
params=params,
)
# unique_together
else:
field_labels = [capfirst(opts.get_field(f).verbose_name) for f in unique_check]
params['field_labels'] = get_text_list(field_labels, _('and'))
return ValidationError(
message=_("%(model_name)s with this %(field_labels)s already exists."),
code='unique_together',
params=params,
)