Well, the idea of one language per country is a really European one. You will hardly find a country outside Europe where there are not at least 3 different languages spoken in. Even in Europe there are several minority languages and different dialects (the borders often overlap). But a lot of them will probably among those 50% of languages that are going to die withing the current century: Frisian (Germany), Sorbian (Germany), Gutamål (Sweden), Occitan (France), Norman (France), Breton (France), several Finnic languages, etc.
The state of having several languages spoken in one single country is by no means a sign of seperation, seen globally it is the normal state. Even for the individual being monolingual is quite rare (but increasing). In most countries you have one or two linguae francae and then several other languages. For example Papua-New Guinea: about 10% of all languages of the world are spoken on that (though big) island. If you assume 6.000 languages, that's 600 different languages. On one island!!! And people are not killing each other. Or take a look at South America: You have Spanish and/or Portuguese as languages nearly everyone speaks plus several hundred different indigenous languages.
You see, there is no need to be afraid or whatsoever of having more than one language in one country. It's no sign of seperation. See it as a kind of cultural diversity. Do you want everywhere to be like the Tuscany? Or Brussels? Or New York?
If there are fights among differnt cultures in one country, you can be sure the diversity of languages is not the reason. But people like to think like that, because apart from skin color and other biological features, the language of another human is the most obvious differentiator.