Ah, I definitely don't think English will be entirely overtaken by Hindu or Chinese by 20 years. In fact, English as a second language won't really ever be completely 'overtaken'. It, of course, depends on where you live and what is most important to you. On the other hand, I do think that in 20 years, Japanese, Koreans and Mongols might have much more of an incentive to learn Chinese than English -- and Pakistanis and Afghans Hindu.
Given time, however, I find it natural that English will be 'replaced', in lack of a better word. Though, again, it won't ever truly be 'replaced': even in a hundred years, assuming the best case for Chinese and the worst case for English, western Europeans might still get a lot from knowing English.
tl;dr: Location, location, location.
As a side note, I'd personally find it really awesome if everyone learned Iláksh or Ithkuil or something. Of course that won't happen... :3