> "At least here in Germany, the apartment owner is allowed to enter your apartment using his key in emergency situations."
I am not a lawyer but I know that this is a common misunderstanding in Germany. The landlord is not allowed to keep a key to the apartment he rented out, unless you're explicitly consent to it.[0]
You have to grant him access in case of an emergency and if you don't do that, forced entry may be legal (like in the case of a pipe burst).