Hi! there are language which are common in the world like Chinese and English but i think your question is about common language in the world then i think it is English because mostly country 's people interact with each other using English language and it is the international language.
The main thing is, are the Americans learning Chinese or are the Chinese learning English. It is safe to say that I do not think many Americans think that learning Chinese is high on their priority list, whereas the Chinese are simply gaga over learning English. Just Google to see how many English speakers head to China to teach English. Verdict: English is set to become the global language.
english is language to have most speaking ( common). And Vietnamese is language special, do you know it???