I use two django packages - django-mptt (utilities for implementing Modified Preorder Tree Traversal) and django-hvad (model translation).

I have a model class MenuItem and I want to it extends TranslatableModel and MPTTModel, like this:

class MenuItem(TranslatableModel, MPTTModel):

but it causes metaclass conflict:

(TypeError: Error when calling the metaclass bases 
metaclass conflict: the metaclass of a derived class 
must be a (non-strict) subclass of the metaclasses of all its bases)

What is the solution this problem? I hope that I can use double inheritance.

You might want to do the following:

class CombinedMeta(TranslatableModel.__metaclass__, MPTTModel.__metaclass__):pass
class MenuItem(TranslatableModel, MPTTModel):__metaclass__=CombinedMeta

This should give you exactly what you want, without any error mesages.