I learned just now, from one of my infrequent visits to the Opinio Juris blog, that Bolivia officially changed its name in 2009 to the Plurinational State of Bolivia, thus formally affirming that it is not a "nation" but a state of several nations, including indigenous peoples. Actually, very few 'nation-states' in the world today are nations in the sense of being composed of just one ethno-national group; most sovereign states are multinational or "plurinational," in fact if not in official name.
The author of the Opinio Juris post, Peter Spiro, remarks that "the nation has generated and justified the state." No, not always. In the case of France, for example, I think it was more the other way around: the state generated the nation. (See Rogers Brubaker's 1992 book Citizenship and Nationhood in France and Germany.)
And what about the coming-into-being state of South Sudan? Plurinational? Well, from what I gather, there are ethnic and tribal divisions, so yes.