old_contents: # -*- coding: utf-8 -*-
  from django.db.models.signals import post_delete
  from django.dispatch import receiver
  
  from .mixins import ModelMixin
  from .models import Translation
  
  
  @receiver(post_delete)
  def delete_translations(sender, instance, **kwargs):
      """
      Deletes related instance's translations when instance is deleted.
      """
      if isinstance(instance, (ModelMixin, )):
          Translation.objects.filter(identifier=instance.linguist_identifier,
                                     object_id=instance.pk).delete()
  
new_contents: # -*- coding: utf-8 -*-
  from django.db.models.signals import post_delete
  from django.dispatch import receiver
  
  from .mixins import ModelMixin
  from .models import Translation
  
  
  @receiver(post_delete)
  def delete_translations(sender, instance, **kwargs):
      """
      Deletes related instance's translations when instance is deleted.
      """
      if issubclass(sender, (ModelMixin, )):
          Translation.objects.filter(identifier=instance.linguist_identifier,
                                     object_id=instance.pk).delete()
  
subject: check subclass not instance.