This concerns the French, German and English versions of WoW:
We wanted to provide the following notice in anticipation of the upcoming launch of the Expansion. Players planning to play the Burning Crusade will need to purchase the same language version as their original World of Warcraft client. The base languages for the Burning Crusade and World of Warcraft must match, regardless of installed Language Packs.
Players who currently have Language Packs installed will not need to reinstall Language Packs after installing and upgrading to the expansion. The Burning Crusade Installer will automatically detect and update Language Packs.