I see that there is a simplified chinese version of the ubuntu official website for people in mainland China but no traditional Chinese version for people in Hong Kong (or Taiwan). If I want to help localize the official website to make Ubuntu more popular in Hong Kong, what can I do?
I don't think cn.ubuntu.com is the result of regular translations by the Ubuntu community. My belief is that Canonical has specialcased simplified Chinese by providing that version of the site. AFAIK there are no localized versions of the official site in other languages; see this bug comment, for instance.
Here's the website localization Launchpad page but I don't see a page for your language here yet.
So you should find some other folks interested in doing it :)
There are some more ideas and links on the Contribute page of the website:
Definitely get in touch with your local team: