One approach Firaxis seems to use for representing large modern countries is equating them with related historical states- so calling Majapahit 'Indonesia' (which seems reasonable enough), and claiming in civilopedia that the Democratic Republic of Kongo is a successor to the Kingdom of Kongo (complete nonsense). Although they don't do that always, Siam is famous in it's own right so they don't call the civilization Thailand in Civ 5. I do agree it would be a bit bizarre to see most modern countries as civilizations. Australia and Canada I could take or leave tbh, especially Canada given it's proximity to America, but what those two do at least have going for them is a relatively high level of cultural unity. Most modern African countries are not plausible as civilizations at all I feel however. Some of the tribes within them and historical kingdoms that existed in those regions would be very interesting additions however.