People from Beijing, China, must be able to write traditional Chinese and speak Cantonese when going to Hong Kong.

Hong Kong belongs to our country, not a "foreign country". Although many people can speak English, many people can speak Mandarin, especially the younger generation. As for traditional Chinese characters, they are similar to simplified Chinese characters. Even if you can't write, you can roughly guess what that word is, so you don't have to worry about going to Hong Kong except for your documents and itinerary planning.