HKUST and HKU and other unis always operate in English. There is English everywhere. All materials are in English.
As for CUHK, because of the tradition, Chinese is more commonly used. Its initial mission is to preserve Chinese culture under the pressure of British colonialism and Chinese communism. However, nowadays, the major teaching medium is English, although Chinese remains as the optional teaching medium in some departments. Surely, Economics is not one of them. It is highly likely that most, if not all, of the modules offered by the department of economics
are taught in English. Even if it is not, I am sure that all modules are assessed and examined in English. All professors are fluent in English as nearly all of them received their phd from foreign unis. They must be able to teach in English, if there are international students in the class. Anyway, you are best advised to consult the departmetn directly.
All universities in Hong Kong except CUHK fully adopt English as their teaching medium. Actually, there are many foreign professors. Using Chinese is as a matter of fact not an option at all! Nevertheless, you should appreciate Chinese. The CUHK was founded by leading and internationally reknown academics who fled from the communist China. CUHK is a place where East meets West, both physically and academically.
According to the Basic Law, English is one of the official languages. Many people here can speak English. There are many many foreigners in Hong Kong!
Also, the CUHK has exchange linkages with Duke, Cornell, Chicago, UC Berkeley, UCLA, Michigan, Georgetown, Williams College, University of Pennslyvania, Columbia, NYU, USC, Boston, Washington, Rice, MSU, Trinity College, Occidental College, London Business School, Glasgow, UCL, Sussex, Toronto, UBC, Mcgill, Australian National Uni, Melborne, Sydney, UNSW. That means that the courses of CUHK are recognized internationally. Otherwise, they will not permit the credit transfer, right?
You will learn a lot about China! Hong Kong is a good place to study Mandarin.
Students in Hong Kong are often fluent in Mandarin, Cantonese and English.
Entertainment is virtually not a problem at all. Hong Kong is a city which never sleeps. I am a HKU (sorry, not CUHK) student. Last semester, I had a Spanish floormate who went to pub everynight! This semester, I had a German floormate who usually played Hockey, an Australian floormate who liked to show his muscle, and a French floormate who watched Chinese TV programmes all day!
Sorry if this doesn't help.