Add custom instructions for de (#2383)

Author: mrubens

.roo/rules-translate/instructions-de.md

@@ -0,0 +1,14 @@
+# German (de) Translation Guidelines
+
+**Key Rule:** Always use informal speech ("du" form) in all German translations without exception.
+
+## Quick Reference
+
+| Category    | Formal (Avoid)            | Informal (Use)      | Example           |
+| ----------- | ------------------------- | ------------------- | ----------------- |
+| Pronouns    | Sie                       | du                  | you               |
+| Possessives | Ihr/Ihre/Ihrem            | dein/deine/deinem   | your              |
+| Verbs       | können Sie, müssen Sie    | kannst du, musst du | you can, you must |
+| Imperatives | Geben Sie ein, Wählen Sie | Gib ein, Wähle      | Enter, Choose     |
+
+**Technical terms** like "API", "token", "prompt" should not be translated.