First of all, you should agree to the terms of the .DE registration terms and conditions.
Denic, the .DE Registry, requires that all .DE domains have a German address for the administrative contact.
.DE domains have different regulations regarding nameservers. A minimum of 2 authoritative independent DNS is required. It means that the IP-addresses of each nameserver should differ by at least one number in any octet. Also, at least one of them must be connected via IPv4.
192.0.2.1 and 192.0.2.2 pair is valid (IPs differ in 1 digit here);
192.0.2.1 and 2001:db8:85a3::8a2e:370:7334 pair is also valid.
You can use Denic's NAST tool to see if the DNS you'd like to use will be accepted for updates or registration.
If you have any questions, feel free to contact our Support Team.