For the on-screen keyboard, you have to remove English QWERTY from the list of available keyboards in Settings-Regional-Keyboard layouts. If you mean a hardware keyboard, you need to install some keymaps
sudo apt-get update
sudo apt-get install console-data
sudo loadkeys fr-x11
Then put a line in /etc/rc.local so it survives a re-boot.
sudo nano /etc/rc.local and add the following line
loadkeys fr-x11
A better way to do this is being worked on.