When the police are at the hotel checking the IDs of the two spies (Peppard and Courtenay), they are speaking German with the innkeeper. At one point she hands an ID to one of the police officers and he responds "thank you". Oops...
The police officer also pronounces the name "Buys" something like "bjew-is" but neither a german or a dutchman would pronounce it like that. Even the Buys character who is supposed to be from Holland doesn't pronounce it right. Come to think of it, the english language simply doesn't have something sounding like that (the "ui" or "uy" sound).
But I liked the dutch angle of the movie, especially since Buys did his ballistic studies in Den Helder, the place where I was born. :-) Den Helder is the dutch naval base, by the way, so that detail made sense.