Without a common tongue the federation is doomed - look at Europe, the lack of a lingua franca is one reason the EC is in trouble.
If you listed all the things wrong with the EU in order of importance I doubt language diversity would make into the top 1000.

But, go on then, if you like. Explain specifically how a lack of a lingua franca has created, or is contributing to, or has contributed to, any of the EU's many problems.