-The game started in German.
-After you change your realmlist.wtf, the game would start as a black non-responsive screen.
--After you change your realmlist.wtf, the game starts, you needed to re-download 1GB of patching each time you start the launcher.
Important: If you have 2 accounts, one for the US, and one for EU, make 2 copies of the world of warcraft folder.
Disclaimer: this solution requires the deletion of files and folders from your wow installed directory. Make sure you backup everything.
1-Locate your realmlist.wtf file inside "Installed wow directory/data/enUS", copy this file, and paste it inside "Installed wow directory/data/enGB".
2-To make sure you don't download additional stuff, copy the interface folder inside "Installed wow directory/data/enUS" to "Installed wow directory/data/enGB".
3-Modify the realmlist.wtf to:
set realmlist eu.logon.worldofwarcraft.com
set patchlist enGB.patch.battle.net:1119/patch
set realmlistbn ""
set portal eu
4-Delete any local folder inside your "Installed wow directory/data/", these are enUS and deEU, delete them or move them into a backup folder away from the main directory.
5-Delete your WTF, interface,cache and data/cache folders, or move them into a backup folder away from the main directory. This is a must for the solution to work.
This should make your world of warcraft locale is set to enGB only, to test this:
Optional but recommended:
6-Start your repair tool, and note the message it gives you, if the "Detected locales : enGB" ONLY, then you are ready to step 7. You may cancel the repair tool once you have the information you need.
7-Start the launcher and download the 2.1 GB patch.