I find it hard to differentiate between people from Hong Kong and mainland China. People raised in Hong Kong since the Chinese take-over were educated in Mandarin since it's the official language. While "forced" to learn Mandarin at school but still preserving Cantonese among themselves, it means they can communicate in either Cantonese or Mandarin. This is according to my Asian wife's relatives from Hong Kong.They all speak Mandarin between themselves so unlikely she is from Hong Kong where they speak Cantonese.