I think the most common languages are Mandarin, spanish and Arabic. They have the most popularity all over the world.

Hm....If be absolutely honest, you don't need to know other languages - everywhere you go, all understand English lang =)

I saw some study about most influential languages all over the world. The first is English, then, I guess, Spanish and then I don't remember. You can try to find the study on google.com.

To my mind, it also depends on where you live. For instance I come from Czech Republic which is country in the middle of Europe and we have a lot of industries within our contry. And one of our neighbor is Germany where are a lot of industries as well. These two contries have a lot of in common then and many businesses are based on Czech-Germany cooperation, due to you need to speak German (not Czech, because Germany is much bigger country then we are). Completely different situation might be somewhere in Asia or even in France and so on and so forth.

Shadi ,

from that aspect , yes you are right , china is rising from the beginnig of 21st century , 

the problem is these language u mentioned are so far harder ( for learning ) than Spanish or German and other European ...

If one 's intention is living and working in the East ( Asia ) yes thats true , 

but about western life I dont think so ( those language are not effective maybe )

Well,I think Mandarin,Japanese or Korean may be the priority after English,based on what we can see is happening in business and job marketing of the contemporary world!