Because Google is still really bad at e-commerce and doesn't distinguish content language from locale and local law. You should be seeing the store in your pref

Because Google is still really bad at e-commerce and doesn't distinguish content language from locale and local law. You should be seeing the store in your preferred language, asking for your locale's address format, and following German trade law. But alas, you are in Germany, you get the full German experience. Also nobody really looks at accept language headers anymore, I feel.