update-grub only looks at $LANG, even though $LC_MESSAGES could be used to choose for example english messages.
/etc/grub.d/00_header - add middle line
grub_lang=`echo $LANG | cut -d . -f 1` if [ $LC_MESSAGES != "" ]; then grub_lang=`echo $LC_MESSAGES | cut -d . -f 1`; fi grubdir="`echo "/boot/grub" | sed 's,//*,/,g'`"