It depends on what you mean by "common". That aside, the answer is of course English. English is not only learned practically everywhere, it is one of the most important subjects at school. For example, in Korea and Japan, far far away from UK or USA, it is one of the two most important subjects, the other being mathematics. In many ways, English not just "important", it is almost a ticket to your success in life. In most professional fields, you have no chance of reaching an elite position if you don't know it well enough. My experience has been that, in the current state of the world, English is more important than all the other languages combined, by far!

Spanish is also spoken by many people. That's why I am learning it.

For me is english. I'm try to improve it for this reason

I am sure Chinese is the most widely spoken language in the world!