Here is a step-by-step list for playing WoW in China (using your own language). I am posting this because it has helped two people who couldn't get the client to work (laminebean and juice), so it must be right. Of course, TBC is coming out soon so it won't matter anymore, but for now, I hope that it can help somebody.
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1. download the basic chinese client
2. create your account at The9, and log in with the chinese client
3. allow the chinese client to fully update until it is finished
4. log into the game with the chinese client to verify that you can play.
5. download the english trial client (from fileplanet), making sure to rename one of the clients' main folders so that you can have both on your computer (example: world of warcraft ..... world of warcraft china)
6. log in with the english client (username doesn't matter at this point), and allow it to start updating automatically
7. After many updates, you will finally notice that the blizzard downloader is downloading version 2.0. at this point, close the downloader and the english client to stop the update.
8. copy the 4 files listed in the forum from your english Wow folder and create a backup copy somewhere else.
9. copy and paste the 4 files from the chinese Wow folder to the english one, overwriting the english ones
10. download the new .exe file, and overwrite the english one.
11. change the realmlist file information (not the filename itself! you have to open the file in notepad and change the contents) to:
SET realmlist "cn1.grunt.wowchina.com"
the number 1 in that is the number of the server, in this case 1, which is shanghai. for those in beijing it is 2, etc.
12. run the modified .exe file

if you follow these steps EXACTLY you will not have any problems. let me know if you still have problems after doing these steps. The only step that you cannot undo is if you accidentally update the english client to version 2.0. If you do this you have to delete the english game and re-download it from the trial client step. All other steps can be undone.

I lock the subject to make sure that nobody post any questions which are already in the other subject.
If you want to add informations, PM me.