Once you have secured your Spanish ISO or DMG:
Open Terminal and paste:
softwareupdate --fetch-full-installer --full-installer-version 10.13.6
This downloads the official installer from Apple’s servers. After completion, the installer is in /Applications.
# Si tienes el ISO convertido a DMG:
sudo dd if=/ruta/al/HighSierra.iso of=/dev/disk2 bs=1m
# Atención: /dev/disk2 es tu USB (identifícalo con 'diskutil list')
O mejor, usa el comando createinstallmedia (si tienes el .app original):
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MiUSB --applicationpath /Applications/Install\ macOS\ High\ Sierra.app --nointeraction