                      id='sites.E001',
                  )
              ]
  
          if not isinstance(field, (models.ForeignKey, models.ManyToManyField)):
              return [
                  checks.Error(
                      "CurrentSiteManager cannot use '%s.%s' as it is not a ForeignKey or ManyToManyField." % (
                          self.model._meta.object_name, field_name
                      ),
 
</s> remove                     "CurrentSiteManager cannot use '%s.%s' as it is not a ForeignKey or ManyToManyField." % (
 </s> add                     "CurrentSiteManager cannot use '%s.%s' as it is not a foreign key or a many-to-many field." % ( </s> remove                 "not a ForeignKey or ManyToManyField.",
 </s> add                 "not a foreign key or a many-to-many field.", </s> remove                         "The value of '%s' must not be a ManyToManyField." % label,
 </s> add                         "The value of '%s' must not be a many-to-many field." % label, </s> remove                         "ForeignKey or ManyToManyField." % (label, field_name),
 </s> add                         "a foreign key, or a many-to-many field." % (label, field_name), </s> remove             elif isinstance(field, models.ManyToManyField):
 </s> add             elif getattr(field, 'many_to_many', False):
        if not field.many_to_many and not isinstance(field, (models.ForeignKey)):
                      id='sites.E001',
                  )
              ]
  
        if not field.many_to_many and not isinstance(field, (models.ForeignKey)):
              return [
                  checks.Error(
                      "CurrentSiteManager cannot use '%s.%s' as it is not a ForeignKey or ManyToManyField." % (
                          self.model._meta.object_name, field_name
                      ),
keep keep keep keep replace keep keep keep keep keep
 Refs #24227 -- Replaced M2M isinstance checks by field.many_to_many

Thanks Markus Holtermann, Collin Anderson and Tim Graham for the reviews. 
django/contrib/sites/managers.py