You're asking about the kindergarten or primary?
As I know,if the school named as International School of course is teaching in English. Like ESF school (primary), they are teaching in english(all subjects) but p1-p3 haveing the Mandarin lesson twice/week.
HK is very strange, there are so many international schools following their own country's education mode, not every one teaches chinese. you should carry out seach in the internet.