Description :

When you launch your game, it is in another language than English.

• Modify the config.wtf or Firestorm.wtf file in the WTF folder and change the following lines: SET textLocale "frFR" and SET audioLocale "frFR" to SET textLocale "usUS and SET audioLocale "usUS"
  
• If the first solution doesn't work, go back to where your WoW.exe is and search after another config.wtf or Firestorm.wtf folder and repeat the solution above.
  
• Also if you use the Launcher, change the language by selecting your expansion, then "Options" and change the language to English.