When I was about ten or twelve, some forty years+ ago, I saw an interview of a captured U-boat commander. His English was so good that I though that he was an actor playing the part of a U-boat officer for the documentary. Many Kriegsmarine officers' English was so good that they could slip into the population of English speaking countries when they escaped and go unnoticed. However local colloquialisms would sometimes betray their nationality resulting in their capture.