Hi Steve,
thanks for your help.
The parameter to select the language worked. But if it is used together with the locale-parameter, the German umlauts are broken. Not so important for me, since I can now execute scripts like ./install.sh without finding the right keys by trial an error (umlauts are rarely used).
Your other hint regarding the partnew-command was good. Although I read about it the evening before my post, I didn't used it because it just looked like a nasty workaround. But I tried it and it worked. It is still a nasty workaround since partnew creates a new partition which overlaps my existing (hd0,0) and GParted and fdisk complains about it. I also have to use different partition-numbers if I want to use this workaround for different ISOs.
A better solution would be, if ESET would support the iso-scan/filename-parameter as ubuntu does.
Here is my now working config:
title ESET SysRescue 1.0.16 (20180521)
# https://www.eset.com/de/support/sysrescue/
find --set-root /ESET-SysRescue-1.0.16.iso
map --heads=0 --sectors-per-track=0 /ESET-SysRescue-1.0.16.iso (0xff) || map --heads=0 --sectors-per-track=0 --mem /ESET-SysRescue-1.0.16.iso (0xff)
map --hook
partnew (hd0,1) 0x0 /ESET-SysRescue-1.0.16.iso
root (0xff)
kernel /casper/vmlinuz boot=casper locale=de_DE.UTF-8 keyboard-configuration/layoutcode=de
initrd /casper/initrd.lz
Tom