Is Castilian the purest Spanish, as well as the accent?
Yes, it is. The homes for purest Spanish with Castilian accent are Salamanca and Valladolid. However, South America has developed what it's called 'neutral accent', which is very similar to the Colombian accent. In fact, perhaps the best author in Spanish is Colombian Gabriel García Márquez (Nobel Prize in Literature 1982). An absolute favorite.
What is the role of the nation-state in International Relations?
The role of the nation-state is complex, but essentially the nation-state created the concepts of citizenship and sovereignty, which enabled the emergence of international relations as a new independent field.
How can I communicate in a culturally diverse society?
Successful communication is beyond speaking the same language. It's also, and perhaps more importantly, about culture, non-verbal communication and perception.