Before you consider that, you’d better define what’s really mean by the “Chinese” language in Hong Kong.
The primary language used in Hong Kong is Cantonese; while the one used in the Mainland is Mandarin (or Putonghua). Cantonese is a much older language than Mandarin and the former is commonly spoken by people living in Hong Kong, Macau and some parts of southern China.
Although quite some Hong Kong citizens speak both languages as Mandarin is widely taught at primary schools, they prefer to speak Cantonese only to show their unique identity – the Hong Kongese-ness.
Some tourists might have no clue about the use of language in Hong Kong and just speak Mandarin there as they think “it’s all ‘Chinese’ anyways. But many Hong Kong people dislike the fact that some foreigners speak to them in Mandarin because locals in general don’t like to be identified as a Mainland Chinese, and the language you speak shows such distinction. The urgency and even desperation for such clear differentiation between different identities can be attributed to some cultural, political and economic issues, such as things from Chinese spitting on and street, to issues like the increasing property prices and invasion of luxury brands in Hong Kong due to influx of tourism dollars from the mainland.
Of course, many locals speak English already, so you can always speak to Hong Kong people in English, and anyone on the streets would like to help you out with directions. Staff at stores and restaurants would be able to assist you in English as well, at least on an elementary level that’s good enough for basic conversations. In business districts like Central and Sheung Wan, people generally speak English well, some are even native English speakers.
Therefore the answer to the question is, yes it would be great to learn some Cantonese phrases before going to Hong Kong, if there’s something you want to choose between the two.
I’m sure if you speak Cantonese in utterly local places like Cha Caan Tang (“tea restaurants” in Hong Kong), the staff would appreciate it a lot.
Otherwise, it’s better to speak English in Hong Kong than Mandarin at all, just so that you don’t irritate the locals.