Hi,
I've come across this problem before.
After you have unpacked MUI check in the Locale/ Languages/Drawer for "French Drawer".
Open this Drawer and confirm that the 2 French Locale files are inside, use "Show All Files" because they are hidden files.
I have found in some MUI packs that this Drawer is empty which causes the installation to fail. Even if you hav'nt asked MUI to install in French.
If you cant find it PM me and I'll send it to you.
Regards, Michael,
aka rockape.