I don't think that Chinese will rival English in the next 50 years. Numbers of native speakers are pretty irrelevant to international language. The boom in the Chinese economy doesn't affect this greatly either- it may be useful to speak Chinese to do business in China, but Japan is the second largest economy in the world and Japanese never emerged as an international language. Therefore, I believe that the economic progress of China and the numbers of Chinese are actually irrelevant when thinking about the language a Brazilian doctor will use when talking with a Vietnamese colleague at a meeting in Canada.
The internet is mostly in English, the language of intenational trade is English. Europe has learned English, so too has much of Asia. Billions of dollars have been spent on training people to speak English. Chinese cannot challenge this international status. I hvae seen arguments that the growth of the Chinese economy will change things, but I think they are wishful thinking- why should a Saudi businessman or woman suddenly decide to learn Chinese in order to fly to Finland to buy mobile phones?
English is an enormously flexible language and the emergence of international forms will change the language greatly, maybe even make it unrecognisable, but no other language will replace it in the next fifty years, IMO.