20-25% of Americans are multilingual. Whether the remaining 75-80% is a "vast majority" is up to you I guess, but Americans by and large don't need to learn multiple languages. We're the most powerful country in the world, we're geographically isolated, we're one of the largest countries in the world, and English is the universal language. There's simply no practical reason Americans need to learn other languages, except maybe Spanish, which most people know a bit of anyways.