Hong Kong's official language is Cantonese, the Chinese dialogue of Guangdong. The use of Mandarin (or Putonghua) China's official dialect is increasingly on the rise, however for now, all street signs and notices are in both English and Cantonese.
The proficiency of English spoken by bus / taxi drivers, tradesmen, service men and security guards alike can vary in levels, so learning a few Cantonese words can help you get around Hong Kong. In the meantime if you are having problems being understood, search your mind for synonyms and use them. Most locals learned their English from the English. So try to think of the British word, for example: boot for car trunk, petrol for gasoline. If all else fails, call a secretary or someone you know for help.
There are many language schools in Hong Kong but today most focus on Mandarin. The YMCA of HK will have regular classes.
Hong Kong Language School
New Concept Mandarin