Almost a fourth in the states speak only spanish, and when they formed the original federation it was far worse. Also large federations such as India, Russia and China are multilingual and multinational.
Most Europeans of smaller countries speak 2 or 3 languages already, only France, Germany, Italy and Spain are the exception to this... oh! and a couple of islands close to Europe
Language barriers and cultural differences IMHO is just an excuse used by conservatives and fascists and their financial supporters... again with the exception to large nationalized countries where the public has limited exposure to the rest of the world.