In Mandrake/KDE, there are two places you can go...
First, if you would want to always have a french keyboard layout, you would have to go in Mandrake Control Center in the hardware section. In there, there is the keyboard icon... You would just have to select a french one! Now be carefull, there are many different french keyboard layout! In particular, you have the one for people in France and you have the one for people in Quebec, Canada... What is the difference? Well the one in France is of type Azerty, which is not compatible with American keyboard. The one in Quebec, Canada, is of type Qwerty, which is, yes, compatible with American keyboards. For your information, just in case you don't know, Azerty and Qwerty are the letters on the keys at the right of the Tab key.
Second, If you just want an additional layout, just go into KDE Controle Center, in the Accessibility->Keyboard Layout section. In there, activate the keyboard layouts and choose the one you want, according to the explanations I have provided above.
Note: The information I have provided is based on Mandrake 10.0 and KDE 3.2. However, it should be very similar with Mandrake 9.2 and KDE 3.1.3...