I think sign language

In my opinion, after English the next global languages to learn are Spanish and French. Mandarin, Russian and German are also very important but with some obvious exceptions are not widely spoken outside China, Russia, or Germany. As with anything, it depends on where you plan to travel or do business.

One recent study was entitled "Links that speak: The global language network and its association with global fame." http://www.pnas.org/content/111/52/E5616.abstract They tried to find out which languages _are translated most frequently._ They found that "the structure of these three global language networks (GLNs) is centered on English as a global hub and around a handful of intermediate hub languages, which include Spanish, German, French, Russian, Portuguese, and Chinese." The idea is that if you write, say, a scientific paper in a language that isn't frequently translated, it might never get noticed. The chances of a piece of work becoming famous and influential are higher if they are in one of the frequently-translated "hub languages."

I mean when I go to another country I can speak one language.I think wherever I go I can find someone speak English.Of course all languages are important but English is common language.I said ''most important'' for trading.

I am not sure what you mean by "most important". All languages are important. The languages spoken by the largest number of people- I expect they are English, Spanish, French, Russian,Chinese. I am not sure really. What is important depends on your reason for learning a language.