The Washington Post has an article asking the question "Which languages will dominate the future?" [washingtonpost.com] The answer depends on your interests: making money in growth markets; speaking with as many people as possible; speaking only one language while traveling; or learning about culture. As you might imagine, the article concludes
There is no one single language of the future. Instead, language learners will increasingly have to ask themselves about their goals and own motivations before making a decision.