I have often wondered how, in detail, agencies managed (and probably still manage) to train agents for covert duty as sleepers in foreign countries, in the language of that country, when the agent was not brought up as a native speaker of that language in addition to his or her own! To avoid detection, these agents must have been able to speak the foreign language remarkably well, as well, in fact as a native speaker - idiomatically, grammatically perfect and with flawless pronunciation and intonation. In effect, they were creating "native" speakers of adults, well after it was thought to be impossible. How did they do it? Does anyone know where to find any information about this, or know of any books that explain some of these secrets?
Almost always these spies were bilingual speakers from birth since they were sons of immigrants. They were obviously trained because even 1st generation immigrants born in another country need an "up-to-date" course (not only linguistic since a country changes a lot in a generation.)
An example, American spies in WWII in Germany were often sons of German immigrants to the US (the same the other way round). All nations have got their share of idealists or traitors, depending on who has to judge them. Not only did they have to speak the language as natives but they also had to look native. I mean it was quite useless is you looked "too Mediterranean" for example to act as a German spy in Aryan Germany even if you spoke perfect German. If the spy was in Russia he would have to look absolutely Slav as well as having learnt the language at home. The same with spies in Japan. How could they possibly look anything but Japanese?
So things were quite more simple than you think.
Sometimes the spy has to be a native of the country being spied upon or has to have spent a great deal of time there. The spy must fit in culturally as well as linguistically.
It's easier for the western countries to be spied upon because its culture is so diverse. The Chinese or the Soviets, for example, could have trained their agents to speak perfect American English (have them listen to radio, TV, and films) and have them sent to the U.S. to understand how our society works. Diplomats and associated staff sent to the U.S. are perfect candidates for being spies, as they are exposed to the language and culture daily. Of course, they're also primary suspects. If they didn't know who played 3rd base for the Yankees in 1964, who would care or suspect them as not fitting in?
For other cultures, such as Muslim countries or some Asian countries, it is far more difficult, if not impossible, for an outsider to learn the traditions and culture or to be able to fake a background as having come from there, regardless of how well they spoke the language.
Granted, immigrants to the US and free western European countries (at that time) would have been amongst the first to train and send to teh East as spies. But what about the other way round? There wasn't an influx of English speakers emigrating to Russia, so the Russians must have developed ways to train selected citizens to become native English speakers. I'll bet those methods are best-kept secret!
I'm sure we could sort all that out if only we had a budget.