defaults write -app Gramps AppleCollationOrder "de"
AppleLanguages corresponds to the language list, AppleLocale to the Format country/Region, and AppleCollationOrder selects the way that lists are sorted. You need not set them all.
To read the setting use:
defaults read -app Gramps
this gave "Can't determine domain name for application Gramps; defaults unchanged" for me; if you have problems you can use
defaults read org.gramps-project.gramps
The settings are stored in ~/Library/Preferences/org.gramps-project.gramps
To remove the special settings