In Mac OS X Server 10.4, the keyboard layout used at the login window on the server matches the installation language, not the keyboard layout selected in Server Assistant. For example, if you choose English in the Installer, and then choose the French keyboard layout in the Server Assistant, login window will use the US Keyboard layout, not the French keyboard layout.
To switch to the desired keyboard layout at the login window, follow the steps in "Mac OS X: How to change the keyboard layout at the login window".