There is no such thing as the Hong Kong Language. The official languages of Hong Kong are Chinese and English; however, the difference between Cantonese and Mandarin makes the answer a little more complicated.
The Difference Between Cantonese and Mandarin
Hong Kongers speak Cantonese, a southern dialect of Chinese originating in the Guangdong region. Cantonese is spoken by Hong Kongers and those in Shenzhen, Guangzhou, Macao, and of course by Chinese people living elsewhere in the world. While there are several other dialects of Chinese, Mandarin is the official dialect of China and is used throughout the country for government communication. Mandarin is by far the dominant language of China. It is also used in Singapore and Taiwan and is the dialect most frequently taught to foreign language learners. The problem is that Mandarin and Cantonese are not mutually intelligible meaning that a Hong Konger can no more understand a Mandarin speaker than they can a Japanese speaker or a Frenchman. So while you may speak ‘Chinese,’ if you’ve learned Mandarin, which is the most popular dialect taught around the world, you won’t be able to use it in Hong Kong.
However, Cantonese and Mandarin do use the same Chinese alphabet, which is what classes them as the same language, although even here the picture is muddy. Beijing and China now use simplified characters, using simpler brush strokes, while Hong Kong, Taiwan, and Singapore have continued to use traditional brush strokes and characters. It is possible for the reader of one set of characters to understand the other, although those accustomed to simple brushstrokes only may find the traditional ones hard to decipher. Find out more in our What Is the Difference between Cantonese and Mandarin article.
English in Hong Kong
English is the other official language of Hong Kong and is often used in legal and business matters. Almost all signs and announcements around Hong Kong will be in English, making it easier for English speakers to get around. Additionally all government officials will have a basic fluency in English as well as most service industry professionals. Most English speakers will be able to navigate Hong Kong with ease but don't expect perfect fluency from every Hong Konger you meet.
How does English fit into the Chinese hotpot of language? Read our Do Hong Kongers Speak English article.