Generate a GRUB keyboard layout file. Below is the command for a french keyboard. For other languages, check /usr/share/X11/xkb/symbols/. Filename choice is not important (you can change bepo).

sudo grub-kbdcomp -o /boot/grub/bepo.gkb fr

Edit /etc/default/grub with root rights to have:

#GRUB_HIDDEN_TIMEOUT=0
GRUB_TERMINAL_INPUT="at_keyboard"

Edit /etc/grub.d/40_custom with root rights to have:

#!/bin/sh
exec tail -n +3 $0

insmod keylayouts
keymap /boot/grub/bepo.gkb

Finally:

sudo update-grub