Nigel gives you the most complete answer you will get. Numerically, more people speak "Chinese" (there are so many dialectical variations that I am not at all sure of either the definition of Chinese (Mandarin?), or of the number for whom it constitutes a first language), but English is spoken across the globe, either as first or second language, more than any other. Second place (in either context) might be Spanish, but it is also, arguably, French.

Given that English, nearly a century after colonialism effectively fell apart, is still the global lingua franca, I think this is probably the most convincing argument for "commonly".