Does post nationalistic mean no more nations or cultural identities? No. Again, cultural identities have existed for eons and they will continue to exist. That being said, humans started off in families, which grew to tribes, then cities, then states, then united states in the form of nations. The next logical step is post nationalism. Short of some extraterrestrial discovery or violent worldwide event, I expect the transition to take a very long time and likely have quite a few bumps along the way. That being said, a proper understanding of ethics should help this process along; hopefully peacefully.
All humans have the same foundation in ethics. Ethics transcends time and place. It has always existed and it will always exist, in the same form; it is non-transient. The only thing that changes over time is personal and group understanding and application of ethics.
While local cultures may solve conventional matters in different ways, and therefore provide great diversity in the forms and functions of life, the application of ethics should provide consistency among cultures provided their understanding of ethics is consistent with every other cultures. Given the relative starting points, I imagine this should take some time as well.