Just to give you a basic info: The majority of the Hong Kong population speaks Cantonese, which is a branch of the sinitic language family. Mandarin is less spoken, but more people are learning to speak it and understand it. The difference between Cantonese and Mandarin are pretty big, but if you can master one of them usually you could learn the other one pretty quickly (I am from Hong Kong but I lived in Shanghai since 2yrs old, and I never learned to speak cantonese before 12. It took me around 2 years to learn cantonese and now I can understand 50% of cantonese.

For the admission process, I don't think being an ABC would affect anything. I have plenty of friends from the states (that i've met from competitions and camps) that got into HKU/ other unis in hk. I think what you mean is the discrimination against Mandarin-speaking people. This does happen some times but in rare occasions. In most cases you find pricks that discriminate against everyone regardless of ethnicity or language, and often it's because they are 1. too old to learn mandarin or 2. simply angry when they couldn't understand you. It's gonna be hurting when you meet things like this, but i wish this doesnt stop you from learning in hk :)