Hi,
For HK mandarin is highly preferred, although at least for BCG there are quite a fewnon-mandarin speakers who are in the HK office though that means you are limited to the clients you are serving and cannot work on projects in China.
For Singapore, you only need to speak English. That being said, a local SEA language is a preference because Singapore is often part of a SEA system.
For more information about Singapore/SEA consulting, feel free to check out my article: