Hi,
I’ll answer in English for the sake of this being an English forum 🙂
The easiest way for this to work is for you to create an additional package with the following files (taken from a German system):
com.apple.HIToolbox
com.apple.loginwindow
.GlobalPreferences
You can find them in /Library/Preferences.
Additionally, you need to add
#usr/bin/env bash
languagesetup -langspec “German”
exit 0
as a postinstall script.
Should work (works in my images).