We will not include DE as a default language for the backend (Interface Labels & Messages), but Sitefinity 4.0 provides UI that you can use to translate all labels in a given language.
From Interface Labels & Messages and you create new labels, export and import language packs.
If you go to Sitefinity >>Administration >> Settings >> Basic >>Languages you can set languages for the backend system
From Advanced settings you can create a policy per role or per users where you can define backend cultures.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items