What is the most important language to learn in this world?

 It is hard to say which language is the most important one. Every language has its own value for existing. But with the development of the world now, we know English language has become a world language, which almost everyone can speak. And also there appears a hot language-Chinese. Now more and more people try to learn Chinese, which is an advantage for job-hunting and others. So I think maybe learning Chinese can be more beneficial for you.

Answer from Jennifer Zhu, Professional Chinese Teacher from eChineseLearning
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I would say English, but it all depends on where you want to live/work. If you live in North America , English hands down, as well English speaking places have better living standards in some cases. As well it is world wide, people in Japan have to take english as well as China.  However the greatest spoken language is Mandarin, but that is more concentrated to a specific area. 
If you are thinking about majoring in a particular language or even just taking it for fun, try Spanish, it will take you far in Europe and South America. 
Mel

Probably English. It's not necessarily the best language, but likely the most important one. At the moment, English is the language most business is conducted with. Chinese is growing in importance, but not as important as English. This is because, unlike the US, people in most non English speaking countries can also speak at least one other language, which most often is English.