When I was looking for a helper, I found in some maids' profile, with experience in Taiwan, speak Mandarin. You may want to find an agency with a pool of maids that have worked in Taiwan.
I agree with this. Go for an Indonesian maid. I have just recruited an Indonesian maid who has 4 yrs exp in Taiwan and she speaks fluent Mandarin. I needed a Mandarin speaking maid as my wife is from China and my in-laws who regularly come to HK do not speak any English. Indonesian maids are usually also very diligent.