Most jobs do require applicants to be bilingual (even trilingual, in some companies), so you can assume it won't be easy. There are opportunities available, though they might be limited to senior positions - where Cantonese/Mandarin isn't a prerequisite.
That said, it would be a good idea to start to getting lessons.
To start your search, try
www.jobsdb.com.hk, or even
www.classifiedpost.com.
You could even try the black and white section of HK Magazine. They usually have a good listing of expat jobs.
Good luck....
