If you choose a keyboard layout in Server Assistant other than the one that matches the language chosen in the Mac OS X Server installer, you cannot switch between keyboard layouts in the input menu (the menu that often looks like a little flag icon).
For example, if you chose English when installing Mac OS X Server, but then choose the French keyboard layout in Server Assistant, when you log into the server after restarting you will have options for both French and US keyboards in the input menu. However, selecting the US flag icon will not change the keyboard layout.
You may now switch between all available keyboard layouts in the Input Menu as desired.