Without a doubt, Chinese Mandarin is the most spoken language in the world due to China's huge population and omnipresent overseas Chinese. However, its complexity, especially in writing, has greatly prevented people of other countries from learning the language.
Under the influence of the rapid growth of China's economy, more and more people, especially young generations, are getting interested in China and its language. As far as I am concerned, for people who want to do business with China or want to find job opportunities in China, of course you will have great advantages if you can speak the language. For people who want to visit China, or get to know a little about China, it's still good to know some Chinese words or expressions.
I agree with most of people here. Spanish is the most popular second language now. But in the future, I don't know. Maybe Chinese Madarin, maybe another language.