Another problem for people just starting to get into German horror are the numerous varying cuts of several Ittenbach/Schnaas/Bethmann etc movies due to the strict German "youth protection" laws.
Best advice for strong German stuff: Bar a few expections (like "Tears of Kali"), skip "amazon.de" altogether and order via Austria, though not all releases are English friendly and some- strangly enough- seem to pop up exclusively in the US, like "Baricade" by Timo Rose (featuring effects by Olaf Ittenbach).
@alex: That list is a good idea, glad to help when needed.
To add a few more:
"Slasher"
"Klischee"
"Ostermontag"
More to follow when my wife doesn't want to get into the net....