Create a dedicated file in the folder /etc/X11/xorg.conf.d:

mkdir -p /etc/X11/xorg.conf.d

cat <<EOF > /etc/X11/xorg.conf.d/00-keyboard.conf
Section "InputClass"
        Identifier "system-keyboard"
        MatchIsKeyboard "on"
        Option "XkbLayout" "fr"
        Option "XkbModel" "pc104"
EndSection
EOF

systemctl restart xdm