Cantonese is generally spoken outside class by local students, Mandarin by mainland students, and English by the international students. Which means that you often have three distinct groups on campus. That is not to say that locals and mainland students won't try to involve you in out of class activities - I think you will find that there will be plenty of students inviting you along and speaking in English so that you aren't left out.
I think all the universities run free Cantonese / Mandarin classes (the HKU ones do look pretty good) for international students. I think they only cover the basics though, so you might want to register for a more formal course such as that run by Chinese University.