The sad thing nowadays is that even though Mandarin is the official dialect of Chinese spoken in China, Cantonese still dominates in hong kong and the other coastal regions of the southeast. Equating Mandarin and Cantonese is like comparing French and German. Knowing one is unlikely to give you any help in understanding the other. Cantonese also just happens to dominate in the traditional film industry there meaning that unless it's meant for wide internal release, the language spoken in almost all martial arts movies is Cantonese. What's funny is that different characters will often speak different dialects so you often have a Mandarin-speaking character or two in the mix. I've never quite figured out why that is since it'd be like seeing an Italian film with one character speaking French and yet all the characters seeming to understand one another perfectly.
One thing I find sort of funny is that while I'm usually able to piece together the dialogue of a kung-fu movie in Mandarin, I can't follow the news at all. It might as well be gibberish and I only catch an understandable word out of 10. I can only assume that's because my education in Mandarin happened when I was just a kid and focused on daily language. There was no reason for me to learn words like assault or robbery or politician. So when the character in the movie says he's going to go to the market and pick himself up some steamed buns, I can understand that almost perfectly while a simple news blurb about someone being mugged and beaten in a laundromat would be nearly incomprehensible. As I've said before, I've probably got the Chinese comprehension of a 3rd grader at best and there are times I think I should try to work on it. God knows that my Chinese has not improved since I was 10 years old probably. Also, while understanding is at least passable, I have almost no ability to produce it on demand. Even simple statements require effort as I have to think about each word and even then there's often a lot of uncertainty. I run into this problem every single time I answer the phone and someone starts gibbering in Chinese. Just being able to say that my mom or dad isn't there at the moment to take the call is a painful experience.
Maybe part of it is just that I don't really have a knack with languages. I took 2 or 3 years of Spanish in high school and it wasn't exactly a fun experience. I've also wanted to learn Japanese in the past but other than assembling a vocabulary of sorts, I've had no success in actually taking it beyond that point. About all I can do is listen along and cheer whenever I actually recognize a word I know. The world would be a much simpler place with a universal translator or two around.