DOWNLOAD VANILLA WOW PATCHES INSTALL
First locate your WoW install directory (this is also where AddOn's and you config files are located).So the second option is a much better choice but does require slightly more work BUT is all laid out step by step below: It is clear how the first option can be done but that does require downloading the complete game again which with all the languages can be up to 30GB.
DOWNLOAD VANILLA WOW PATCHES DOWNLOAD
Download a fresh copy of WoW using the "Full Game Client" mentioned above and then select all the languages you want when prompted.There are however two options available to you: Unfortunately blizzard hasn't yet released the language packs for V4. If however you only chose one or two languages from the above mentioned options screen and later wanted to install a further language then you would want to use one of the "European Language Packs" on the aforementioned download page. Tip: If you only want to change the default language (so only one language is installed and it isn't the current default) you can click the "(Change)" link under one of the two installer download options mentioned above and from then on all the download links will change to the specified language.