Mandarin is the official language of China, but there are scores of dialects spoken. Some are so different that that are mutually unintelligable. The two main languages are Mandarain and Cantonese. Most Cantonese speakers are in the south and understand a bit of Mandarin. Mandarin and Cantonese use the same same characters, so something written can be largely understood by both. If it were me, unless I was going to live in the southeast or Hong Kong, I would learn Mandarin. Good luck!