In all likelihood the system will handle this automagically if a Chinese input method is selected by the user. Trying to implement your own system for dealing with Chinese text is probably a bad idea.
Be sure to test it on a Chinese version of XP before deploying, of course, but with any amount of decent design in WPF, and XP as a whole, at all; it should Just Work.
Not really an answer to your question, but as far as I know, this is handled by the operating system, and I'm not sure that application really have that much control over it.
Here's a guide on how you enable pin yin Chinese character entry in Windows XP:
My application works with chinese input, and it is using WPF, i don't get what you want to ask. You can input chinese text in a WPF textbox using sogou pinyin, or microsoft pinyin. Tested in xp and win7
It is just because your ime is setted to input in english. Press shift once briefly if using sogou pinyin and you will see the hanzi of "yi" become "zhong" At that point you will be able to type in chinese ;-)
Also, there are IME property on textboxes, select to have it On, otherwise the IME will be in "english mode"