How might language to divide people and prevent them from forming their own country?

The language is the most basic mean of understanding and collaboration between the people. If there's two or more groups of people that speak different languages, than automatically they will only communicate and collaborate with people that speak the same language, while avoiding the people that speak another language as they will not be able to understand each other.

This can be a very big reason for division between the people. This usually means that there will either be a country that is divided and functions with two or more confederate states, or more likely it will lead to separatist movements and creating two or more nations. The borders will most likely be set in accordance to the areas where certain language is spoken.