It'll revert you to a previous patch. I was reverted last night when I did it. So, we have to patch the client correct? Well, the enUS ones won't work if they find other locals in the data directory.
** this is as far as I got last night **
What you need to do is go download a patch for the EU client and install it to get your system patched up.
Now, when you go to the launcher, and select options you have localization choices.
I'm not exactly sure what to do when a new patch comes out. I would assume that using a EU patch would work, however I can't be sure.
This post is based off of a discussion on IRC during late night (early morning) conversation. It may break your wow install as I haven't had a chance to fully finish it off, plus the fact that it was late at night when this was talked about. :P