KCIS, the international section is taught in English with a large amount of Mandarin everyday.
The KCIS teachers are usually native English speakers; and then the Mandarin teachers take the Mandarin classes.
As the main section of the school teaches only in Mandarin, there is still a big emphasis on Mandarin in the international section.
Some children go to main section in the morning and then international section in the afternoon. Others choose one or the other. Either way Mandarin plays a big part in the school.
From my experience of esf kindergartens, and this is from 2-3 years ago, there was very little emphasis on Mandarin. But it may have changed.