What language to speak when traveling to Hong Kong
The native language spoken in Hong Kong is Cantonese, and writing is in traditional Chinese characters. Besides Cantonese, about half of the locals also can speak Mandarin, although it's not used much in everyday situations unless dealing with mainland tourists and business people
You can imagine that, as a former British colony Hong Kong has high English proficiency level. However, high does not equal universal. There are definitely many locals who barely speak the language. After 1997 English has stopped being the main language of instruction in schools
But as a tourist, you'll have very little trouble getting around the city exclusively in English. Firstly, all public signs in Hong Kong have English on them, so you'll have no problem whatsoever following the signs.
In terms of communication with other people, everyone you'll deal with in official situations (e.g. airport, MTR stations, train stations), as well as tourist businesses (e.g. hotels, the various theme parks and tourist attractions) speak excellent English with very easy-to-understand accent
Geography also plays a role: if you're in Central, Mid-Levels, and Tsim Sha Tsui, a large portion of people speak good English. Outside of these areas, especially on the Kowloon and New Territories side, then it becomes hit or miss
Overall, I'd rank Hong Kong below Singapore in English proficiency, and if I had to guess I'd say 35% of HKers speak fluent English, 35% know enough to respond to basic tourist questions, and 20% know almost nothing
Having lived in Hong Kong for a little over a year, these are the places I've noticed where English skills are lacking and you may have to use Google Translate:
Taxi drivers: many taxi drivers don't speak good English, which is ironic because they deal with tourists so often. This applies to Uber drivers too; I'd estimate about half of the drivers I had did not speak much English
Very "local" restaurants, supermarkets, and shops: I don't mean the fancy ones or the chain ones in the malls. But if you go to "local" neighborhood (like Mong Kok, Causeway Bay, Sheung Wan, Tsuen Wan) that looks like this:Then you'll very likely be dealing with waiters and owners that don't speak much English. In cases like this, bring your phone and have Google Translate ready to go
One thing though is that some of the most basic words in English, like hello/thank you/sorry, are almost universally understood. So at least you can greet people with English. Although to be fair, for these simple words you should definitely learn to speak in Cantonese!