Cantonese！It's used widely in china!

不管是普通话还是英语，只要是真诚的语言就可以，做生意诚实守信是首要条件！

hello Defjon~ i think it depends on the areas you do business in and people you do business with. except English,in some places like Xinjiang,Inner Mongolia,and the Northeast, many people choose to learn Russian. there're also many people in the Northeast,Shandong,Tianjin and so on prefer to learn Japanese and Korean,because there're many Japanese and Korean industries in these areas.besides,some Chinese people also are interested in French and German.

其实在中国每个地方的语言都不一样，如果在南方做生意的话，粤语比较好，北方普通话就可以了。我个人认为，作为一个外国人，在中国会说普通话就很不错落，呵呵。

cantonese