How do/did spies learn foreign languages?
Hopefully I don't need to explain my question in more detail.
Something which caused me to wonder about this was seeing 'Inglourious Basterds'. The British soldier is 'caught out' because he uses the hand sign for the number '3' in a way that wouldn't be used by a real German person.
However, he's also questioned about his 'unusual' accent.
I assume, though, that there have been many spies who have been successful in fooling natives.
Especially in the days before Internet, how could they have succeeded? Or do I just have it all wrong, and the only people who could be spies were in fact those who were already bi-lingual?
I bet this is a really naive question, but I agree with Jason, the only stupid question is the one you never asked.
8 Answers
I suggest that you read Chapter 12: 1-6 of the Book of Judges (the shibboleth account) to see what happens when you can't quite pass as a native of the region. If you're not familiar with the story it is analogous to Spanish soldiers fighting English forces and making their password "ferrocarril". Most English speakers could not trill the rr's correctly so they would immediately be recognized as the enemy.
ephthah and Ephraim 1 The Ephraimite forces were called out, and they crossed over to Zaphon. They said to Jephthah, “Why did you go to fight the Ammonites without calling us to go with you? We’re going to burn down your house over your head.”
2 Jephthah answered, “I and my people were engaged in a great struggle with the Ammonites, and although I called, you didn’t save me out of their hands. 3 When I saw that you wouldn’t help, I took my life in my hands and crossed over to fight the Ammonites, and the LORD gave me the victory over them. Now why have you come up today to fight me?”
4 Jephthah then called together the men of Gilead and fought against Ephraim. The Gileadites struck them down because the Ephraimites had said, “You Gileadites are renegades from Ephraim and Manasseh.” 5 The Gileadites captured the fords of the Jordan leading to Ephraim, and whenever a survivor of Ephraim said, “Let me cross over,” the men of Gilead asked him, “Are you an Ephraimite?” If he replied, “No,” 6 they said, “All right, say ‘Shibboleth.’” If he said, “Sibboleth,” because he could not pronounce the word correctly, they seized him and killed him at the fords of the Jordan. Forty-two thousand Ephraimites were killed at that time.
1 Los hombres de Efraín se alistaron, y cruzaron el río hacia Zafón y le dijeron a Jefté:
—¿Por qué fuiste a luchar contra los amonitas sin llamarnos para ir contigo? ¡Ahora prenderemos fuego a tu casa, contigo adentro! 2 Jefté respondió:
—Mi pueblo y yo estábamos librando una gran contienda con los amonitas y, aunque yo los llamé, ustedes no me libraron de su poder.3 Cuando vi que ustedes no me ayudarían, arriesgué mi *vida, marché contra los amonitas, y el Señor los entregó en mis manos. ¿Por qué, pues, han subido hoy a luchar contra mí? 4 Entonces Jefté reunió a todos los hombres de Galaad y lucharon contra los de la tribu de Efraín. Los de Galaad derrotaron a los de Efraín porque éstos les habían dicho: Üstedes los galaaditas son renegados de Efraín y Manasés.»5 Los galaaditas ocuparon los vados del Jordán que conducen a Efraín, y cada vez que algún sobreviviente de Efraín decía: «Déjenme cruzar», los hombres de Galaad le preguntaban: «¿Eres de la tribu de Efraín?» Si él contestaba: «No»,6 ellos decían: «Muy bien, di "Shibolet" .» Si decía: «Sibolet», porque no podía pronunciar la palabra correctamente, lo agarraban y allí mismo, en los vados del Jordán, lo degollaban. En aquella ocasión murieron cuarenta y dos mil hombres de la tribu de Efraín.
Interestingly enough, I met an ex-Soviet military intelligence officer who can speak English with an Oxford accent when he wants to, although he usually speaks English with a strong Russian accent. He learned through an intensive training program, and speaks ten languages. They also learn to speak their native tongue with a foreign accent.
When I studied Arabic at the Defense Language Institute (aka "The Army Language School) we had a few spooks taking classes. Actually, they were not officially spooks but there was a sprinkling of students who dressed in civilian clothes and among them some said they were with the FBI while others who wouldn't say what their affiliations were.
Most of the students were military (all branches) and were starting a language from zero but the facilities certainly existed for more advanced courses. Another possibility would to have sent the most promising students for further training somewhere else.
Obviously government agencies would prefer to use bi-lingual speakers where possible since they are usually more competent speakers (besides avoiding the expense of sending them for language training).
I spoke French as a little girl, but sadly through lack of practice I can't say very much these days. There are tapes of me speaking French, though, when I was about 3 and I don't think I have a Scottish accent. And the few things I can now say in French sound pretty convincing (at least I think they do!).
I've also spoken Spanish for about 13-14 years (since I was 13) and consider myself bilingual as we speak both languages at home and work (although my English is certainly stronger). It was very interesting to read above that "ferrocaril" was used as a password, because after nearly 14 years of speaking Spanish it's still that "irr" sound "que no me sale" sometimes!
Hopefully one day I'll master it and I'll be able to pass as an auténtica Limeña! hehe 
In a country as large as The US and with such a cultural mix, the intelligence agencies recruit intensively people who were born and raised here, but have maintained fluency in the language of their parents.
Well, back in my spying days...no, just kidding.
Interesting question! And wouldn't it be nice to be able to find a fast (and easy) method. Some people seem to learn languages soooo easily, I believe that after they know more than two somehow they learn 'a method' or something and they just seem to learn more easily. I have a nephew who knows five languages (sigh) - English was his fifth and it is absolutely flawless - I listen hard when he speaks and he just never messes up - and he's a baby - (in his twenties). I guess for we who struggle it's just better not to worry too much. Re the spys though - I think that governments look for people like my nephew and recruit!!! 
The U.S. Military has a school in Monterey, California called the Defense Language Institute. Anyone in the military who needs to learn a foreign language is assigned there. My father (Navy) attended to learn Spanish before we were assigned to Venezuela. His was a 6 month school, 8 hours a day, 5 days a week, with 2-3 hours of homework every night, including listening to tapes. The Mandarin course I believe was 2 years. Some of the Asian language courses taught only to listen, not speak or read/write. You can guess what those guys did for a living.
I assume all countries have something similar. I don't know if the civilian intelligence services used the school, but I don't know why not. It's a great school if you can handle taking a drink from a firehose.
