Cantonese is the second most widespread Chinese language with 70 millions speakers (still more than Italian). It is spoken in Hong-Kong, South-East China (region of Canton) and in many overseas Chinese communities. This is the only other Chinese language that you might want to learn, except if you have an interest for a very specific region.

One of the problems of learning Cantonese is that it is not unified as Mandarin is. When the number 70 millions speakers is mentioned, it includes very different version of Cantonese that are not mutually intelligible. People from Hong-Kong and Canton can understand each other but if you go deeper in the countryside, the dialects are more and more different.

Concerning the written system, Mandarin and Cantonese use roughly the same characters. However, some characters are specific to Cantonese and the grammar can be different. It can thus be difficult to read a Cantonese text if you are used to read Mandarin. Newspapers in Hong-Kong are written using standard written Chinese, which is based on Mandarin. Only popular newspaper use specific Cantonese expressions.