Yuuki said:
All those saying "English!", no thank you. English is an absolute MESS of a language and probably ranking in some of the most difficult languages to learn in the world. To a non-English speaking person, it's is an absolutely nightmare to learn and nothing makes any sense since there are so many exceptions to rules and new words being added to the dictionary every damn day.
I would recommend something far more "stable" and easy to pick up (verbally at least), e.g. Japanese.
The problem with Japanese as an international language is that
no one outside of Japan speaks it. And as much as I like living in Japan, it's just not essential in practical terms for the world at large to speak with Japanese people on their own turf. Being able to debate the nuance of feeling one experiences while looking at cherry blossoms in Japanese may be very gratifying for some, but it's not going to get you a better job or secure an important business contract. Besides, I think most Japanese people wouldn't want their language to be the world's lingua franca. There is a certain amount of discomfort from many people here whenever non-Japanese people are able to read into Japanese discourse and understand what's really being said. Take for example, the Daily WaiWai, a now-defunct website that translated salacious gossip from various Japanese tabloids into English. These tabloids are all over the place, and Japan is filled with racy stories in magazines. But the moment a group of Japanese people got wind that these stories were being translated and were potentially making Japan look bad to foreigners, they made a petition to get the website shut down. And they succeeded, since the site was hosted by a Japanese news agency rather than an organization abroad. While it's by no means universal, there is a segment of the Japanese population that wants very badly to keep foreigners and the Japanese language separate.
Which winds us all back around to my original point. It's great if you want to study Japanese. There's just no point in anyone declaring a lingua franca. These things arise not because we will them into being, but just by shifting masses of people converging on one opinion.