People who speak Cantonese, mostly can understand Mandarin. Because Mandan is a required course in school and it is the official language of China.

But if people who speak Mandarin mostly cannot understand Cantonese.

Thanx for your input Sean.....that's very interesting.

Who knows, maybe after I conquer Spanish, I will try Cantonese or Mandarin, lol.

I am the person you are talking about.

I can not only speak Mandarin but also Cantonese.

In Guangdong Province and for our generation, most of us can speak and understand both of them while for some older generation, they cannot understand and speak Mandarin, they just can say Cantonese.

In other provinces in China, I think they cannot speak and understand Cantonese. But they can speak theor own dialects. Cos there are mumerous dialects in China.

And for Cantonese, it also includes many of sub-languages. For ecample Cantonese in Hongkong and Cantonese in Zhanjiang( a city in Guangdong) are totally different.

They are different phonetic languages. In Hong Kong, (outside of mainland).  There are many Hongkongers who don't speak Mandarin and they use the traditional character. In the south part of China, We have to learn it in school. So we can speak both. but out of Guang Dong province(and Guang xi). if they want to speak Cantonese they have to learn it and speak with an accent. but the characters are same as simplified in China.