Which one?
 

Do they speak Mandarin or Cantonese in HK?
Which should I learn?

Cantonese mate, but you will get away with English in alot of places. After all it was a former colony. But you may find some of the standards of English a bit low, but most of the time English is fine.

Shouldn't worry about learning the language, I speak cantonese, but then again I've lived out there since I was 3. It's quite difficult to learn from scratch. But knowing some greetings and how to say please and thank yous won't be a bad idea. But you'll pick that up when you're there. And most expats in Hong Kong only speak English.

So don't worry

BTW

BTW

Thanks for that.

I thought so since it is the languauge of the area. And yes it is a bit difficult, but not too dissimilar to Bahasa Indonesia.