Cantonese is the language of Hong Kong and Mandarin (or Putonghua) is spoken in Mainland China. English is also spoken in Hong Kong although there has a been a growing tendency to cater more to the local population since the handover. As an example, stickers are now being placed on English products that now explain the ingredients or composition in Cantonese.

Mandarin is taught in many of the International schools and many believe that learning Mandarin will equip their children well in the future with some people maintaining that Cantonese will die out over time. Nonetheless Cantonese is the most widely spoken Chinese dialect outside of China. I would recommend that you enrol in a local course run by the YWCA/YMCA/British Council or private teacher and learn a bit of the local language - at least as much as you need to get by.