ubuntu-server.seed :

d-i debian-installer/locale fr_FR
d-i console-setup/ask_detect boolean false
d-i keyboard-configuration/xkb-keymap select fr
d-i mirror/country string FR

grub.cfg :

menuentry "Install Ubuntu Server" {
        set gfxpayload=keep
        linux file=/cdrom/preseed/ubuntu-server.seed quiet auto debian-installer/country=fr debian-installer/language=fr debian-installer/locale=fr_FR.UTF-8 /install/vmlinuz keyboard-configuration/xkb-keymap=fr  ---
        initrd  /install/initrd.gz debian-installer/country=fr debian-installer/language=fr debian-installer/locale=fr_FR.UTF-8 keyboard-configuration/xkb-keymap=fr
}