If your searching for a french version of IE 8 RC1 in 64-bit you can forget it. I don’t know why, but Microsoft didn’t publish any (only a 32-bit).
You can’t install the 32-bit version on your Vista x64, so you’ll have to install the 64-bit english version from Microsoft Download.
Strangely enough there was a french version of IE 8 Beta 2 in 64-bit. What is the difference between Beta 2 and RC 1 that made MS drop the french in 64-bit ?
By the way if you had any scripting problems with BETA 2 (InternetExplorer.Application object), I strongly recommend upgrading to RC 1.