Remark: after patch 4.1.0 (13914) and using the old tweak settings, I realized my US WOW will auto change to Chinese text. After several tweaking, I managed to get it work again - follow c) & d) exactly. Have fun!

a) Download US Full Game Client from
https://us.battle.net/account/download/

b) Patch US Full Game Client to latest from Launcher ie 4.1.0 (13914)

c) Update following line in World of Warcraft\WTF\Config.wtf
- SET locale "enUS"

d) Update following line in World of Warcraft\Data\enUS\realmlist.wtf
- set realmlist tw.logon.worldofwarcraft.com
- set patchlist enUS.patch.battle.net:1119/patch
- set realmlistbn ""
- set portal us

e) Optional: Install chinese font to wow folder so can display in-game chinese text (to World of Warcraft\Fonts folder)
- Downloadable from http://www.gamefront.com/files/17325058/Fonts.rar/

f) Optional: Install Google Pingyin to type in chinese (press ALT+CTRL during in-game to toggle English/Chinese input)

Reference
https://wow-in-chinese.forum2jeux.com/t1533-taiwanese-world-of-warcraft-just-for-add-in-guide

ps: Am from Singapore & looking for English speaking guild on Taiwan 冰霜之刺 server, do PM me if you know one, thanks alot!

Quote :

f) Optional: Install Google Pingyin to type in chinese (press ALT+CTRL during in-game to toggle English/Chinese input)

Correction:
f) Optional: Install Google Pingyin to type in chinese (press ALT+SHIFT during in-game to toggle English/Chinese input)