Servus,
there are only two locations left in Europe (the others have been closed by the FAA) where you can take FAA written tests, Paris (Le Bourget Airport) and London (Farnborough Airport) the tests are given by Flight Safety International, testing fee is 195 Euros. I just took the FAA ATP written myself last week in Pairs, so if you have any questions feel free to drop me a pm.
Here a few links containing information regarding FAA written tests in Europe:
FAA Test
Europe | FlightSafety International Inc.
Cheers