Cantonese or Mandarin speaking. They are very difficult languages, so don't get discouraged if not picked up quickly.
They are not allowed to do door knocking, but are allowed to do street contacting. One of the restrictions is that if they are teaching someone or a member goes back to Mainland China, the missionaries are not allowed to contact them while they are mainland.
Bring a full mission's supply of deodorant and toothpaste. They are the most recommended and requested items from missionaries serving here.
Amazon is great to send most other needs that they can't find there. It is free shipping on most items if the order is over $49.
Allergy medication is very difficult to find there. They sometimes carry it in American stores, but it is hit or miss.
It is hot and humid most of the time, with a humid cold in the winter that can be quite biting. A lightweight rain coat or just an umbrella will do, and a coat for winter.
You can buy most things there for pretty cheap, so no need to overpack.
During a large part of the year it is hot and humid, so be prepared to feel sticky. Sisters often prefer to wear light dresses rather than pants.
Bring COMFORTABLE shoes, waterproof preferably.
Have a suggestion or correction for the China Hong Kong Mission? Submit it here.