I once knew a family, who had two cars: one for him to drive to work, and one for her to drive to work.
Then they got really really rich, and both stopped working, and got rid of one of their cars because they take helicopters everywhere instead.
One day they met Akuperma. He made fun of them for being so poor they only have one car.
Americans only know one language because we have made our culture and language so dominant that there is little need to know any other language anymore. And where there is need, we pay someone else to know it for us.