Cantonese is one of the languages spoken in China. It is not a dialect of the invented "Chinese language". It is a language itself, such as Mandarin. The only difference is that Mandarin is the official one, and Cantonese is not.
Luca: Hi, Zeke! what've you been up to?
Ezekiel: Oh, I've been studying a lot for my exam tomorrow
Luca: exam of what?
Ezekiel: Oops, I forgot to tell you that I took up Cantonese, which is a language spoken in China. It is not the official one, though.