English, without question. 

I don't know why some people say Chinese - I have never met one person studying Chinese (in person, I mean; of course if you go to a website you find them, like on this website), much less is it a trend anywhere where I have ever lived or visited.  But I have met so many people studying English.  Even on a website like this, there are many more Chinese people studying English than there are people studying Chinese, and that is why many Chinese complain that it is so hard to find partners for language exchange.  That tends to be the case with all the languages, I think.  Also, I can't think of any non-Asian country where Chinese is taught as a required second language in schools, whereas in many countries all over the world English is required as the second language in schools and universities.  Various countries teach university courses in medicine and IT only in English.  English is the language that everyone is studying (unless they speak it already), so it is not hard to guess what will be the most common and thus most important language a few years down the road. 

Sometimes people say that the fastest growing language or the fastest growing religion is this or that - but usually those supposed facts are based on immigration of people who already speak that language (or already have that religion) into a new area or country and bringing their language/religion with them, rather than people newly acquiring that language or religion.  So those numbers are deceiving - they are just immigration numbers, effectively.