code: def _forwards(apps, schema_editor):
      
      # Need to work on the actual models here.
      from fluent_contents.plugins.markup.models import LANGUAGE_MODEL_CLASSES
      from fluent_contents.plugins.markup.models import MarkupItem
      from django.contrib.contenttypes.models import ContentType
  
      ctype = ContentType.objects.get_for_model(MarkupItem)
  
      for language, proxy_model in LANGUAGE_MODEL_CLASSES.items():
          proxy_ctype = ContentType.objects.get_for_model(proxy_model, for_concrete_model=False)
          MarkupItem.objects.filter(
              polymorphic_ctype=ctype, language=language
          ).update(
              polymorphic_ctype=proxy_ctype
          )
docs: Make sure that the MarkupItem model actually points
      to the correct proxy model, that implements the given language.