Not only interview. If in IS must speak english everwhere. My reasons to put my son in IS: want to have a happy childhood without stress, develop an outspoken charcter. Hope this help.
韓國國際(KIS) international section interview with English (at least the present or former principal are non-Asian and do not (or suppose cannot) communicate with Cantonese.
Yes, as long as you can communicates with teachers, western people are very nice, they don't care grammar too much. Also, if your kids can understand the basic instructions, I believe it is not that worst. Children learn new languages are too easy if they are very young. If you can try, the interviewers will tell you whether he/she can fit into their schools.