Though English is one of Hong Kong´s official languages, most of the locals speak Cantonese, which is a dialect of Chinese.
There are two main dialects of Chinese: Cantonese and Mandarin. They are tonal languages, which means that the way in which various vowel sounds are pronounced alters the meanings of entire words. Mandarin is said to have four tones, while Cantonese has nine. Though Cantonese is most common in Hong Kong, increased contact with mainland China has led to the appearance of Mandarin speakers in greater numbers.
Cantonese or Mandarin?
Which of Hong Kong´s two Asian languages you should learn will depend on your lifestyle in Hong Kong. If you will be spending most of your time in Hong Kong interacting with locals, you will definitely want to learn Cantonese. Knowing Cantonese will integrate you into your surroundings, help you navigate the city streets and manage your daily life, not to mention impress the locals that you meet.
However, if you are a business-person who will be travelling to mainland China or interacting with mainland Chinese on a regular basis, you will probably want to learn Mandarin instead. Of the two languages, Mandarin will certainly be more useful in your business dealings, though you will also most likely encounter Mandarin-speakers in Hong Kong as well.