This often comes up in movies, but I can hardly find any real cases, except for the "Illegals Projects" - but that doesn't seem like high-stake espionage, let alone successful.
I'm thinking specifically of cases where the spies had to hide their substantial knowledge of an entire other culture, being fluent in the given language, etc., e.g. when they come from another country (and they pretend to be either native to the country they spy on, or to have come from a third unrelated country).
Any examples are appreciated, especially with a link/reference to the source where the details are described.
(You are welcome to add any examples as answers. I guess this is not a question that has a very definitive answer but I'll upvote any good examples.)