>Now I hear people saying things like, 'yeah we went through this already with Japan, and the US is still the largest economy'.
It's not just Japan, it happened with the Soviet Union, Germany, Japan (and other Asian countries with fast growing economies), The EU, and now China. If you haven't read it before there's an old Krugman article that's relevant.
http://www.ft.com/intl/cms/b8268ffe-7572-11db-aea1-0000779e2...
When I was a kid in the 90s we were all terrified of the talk that we'd have to go to school on Saturdays to keep up with the Japanese. When my parent's were in school they were worried about the Soviet Union and their unstoppable command economy.
People have been predicting the end of US dominance since the beginning of US dominance. It will happen at some point, it's inevitable. Obviously the fact that China has more people than the US means that eventually, all other things being equal, they will have a larger economy.
However, they cannot keep up anywhere near the growth rate they've enjoyed over the past decade, which means it will take significantly longer for their economy to overtake ours than is popularly perceived.
In the time span we're talking about, who knows what may happen--what new technologies may be developed. That many extra people may prove to be less of an advantage with the increases in automation we'll see over the next 50 years.
My point is that history shows that basing our future decisions solely on what seems "inevitable" today, is folly.
In summary, don't worry about learning Chinese because you'll have to know it to transact with our new overlords in 2050--come the middle of the century, you might realize it wasn't necessary after all.
Learn Chinese because you want to learn it, because you find it interesting, or learn Spanish, or German, or Japanese for the same reason.