Are there any specific requirements for registering a .DE domain?
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. The .DE domains have different regulations regarding nameservers. A minimum of 2 DNS is required, and both cannot be located in the same class C IP address.
You can use Denic's NAST tool to see if the DNS you'd like to use will be accepted for updates, or registration.
For example, if you want to use your own DNS, you must enter two nameservers where the IPs are not located in the same class C. Otherwise, the .DE registry will not allow it:
dns1.sample.com 96.123.64.40
dns2.sample.com 96.123.64.45
The 96.123.64.xxx is the same class C.