The answer is in  Genesis, Chapter 11.

Everyone doesn't speak the same language because everyone has different cultures and borders. Languages are ever-evolving, just like people. There are lingua-francas, like English, that have enough culture and influence that they affect how everyone in the world communicates; however, even english is broken down by different accents in different regions. This is due to the separation of people over time. With enough time, those accents soon become different dialects and then eventually different languages. The advent of technology in the information age may slow that down, but as long as we share different communities and slangs, we will eventually have different languages, even if the world spoke one language. 

Language reflects and shapes culture massively.  It would be a huge political decision for any country to change its main language or adopt a second main language, with long-term cultural consequences. In my view, those who want such a change and those who don't, should be free to lobby democratically for their position. 

there's absolutly different education systems in the world that cannot gather all needed knowledge and get it us. But one confederation with particular law can perform that even if will humans of different culure groups. Yaroslav explained.