If you can't speak Mandarin or Cantonese fluently, you can speak English in Hong Kong. I believe the waitress , the salesman or most local people in the street can chat with you in English.

You can speak Mandarin in Hong Kong. Whether they understand you or not is another matter and they would probably revert to using English if you are more fluent in English. I wouldn't expect a local Hong Kong person to reply in Mandarin unless they absolutely are sure you can only speak fluent Mandarin (and have zero proficiency in English or Cantonese). It's partly due to the city lifestyle and fast paced lifestyle.

I would say you are better off just learning Mandarin and learning a few Cantonese phrases just for fun. One type of Chinese is hard enough. Why split your time with two and slow down your language acquisition?

That's what i am worried about, two languages at the same time.....

So is should stick to mandarin?

Ali, it’s great if you’re capable of learning these two languages which are totally new to you at the same time.  But don’t you worry that you will confuse them, do you?

In my opinion, it’s not necessary for one to learn Cantonese just for going on holiday in Hong Kong.  People around business districts and tourist attractions generally can speak and understand Mandarin.  But don’t expect that each of them has a good command of it.  Finally, will you be understood then?  To some extent, it also depends on how well you can speak it?:-D

@Ali,

I am only now learning Mandarin. I already speak Cantonese as a second language learner. :-)