I have seen the installer of WotLK on a german games news page and it looks like they indeed included all european language versions onto the DVD.
Wether it will be the language pack or the "real" version (enGB instead of deDE), we have to see.
I am in the same position. When WoW was first released I was in Great Britain and bought my copy over there (engGB).
For BC I got myself a enGB copy via ebay. Now that I live in Switzerland I preordered at Gamestop.
I hope that WotLK really includes all language versions or that I can just patch the beta client.
Because you can not install a deDE version ontop of the enGB version of the game, that is the problem.