Hello Christian,
I do not thing it will be possible with SAP Standard settings. You will have to speak to your Developer to explore the possibilities if enhancing the screen layout.
This will also involve taking Access key from SAP, as you will be modifying the SAP Standard object.
But what is the business requirement to have country key in the Sales Document Partner Tab? User can anyways see full Customer address by double on respective partner.
Thanks,
Jignesh Mehta
