I also know that he spoke four languages: Russian, English, German, and Spanish. He never once talked about his mission/his work, only that he was stationed in Berlin. Even his wife never knew anything, or if she did she never spoke of it to anybody else. Is most of the work we did at the time in the US still classified, or could someone here shed more light on the subject?
Hello, and first of all, sorry for your loss. Let me first start by saying that I have very little background information here to go on.
First of all, he was a ''spy for the US government''. This is not very clear to me what you mean by that (did he give any more specific information?). ''Spy'' is rather political or literary language, in intelligence there is no such a function. Usually when people mean spies, they mean what in intelligence circles was called an ''agent''. Their handlers, ''case officers'', work for an agency, like CIA or KGB etc.
So what I'm left wondering now is whether he was an agent recruited by the US intelligence in Berlin, or whether he was stationed in Berlin as a US intelligence officer. His nationality also would clarify something.
In general, spies were the infantry of the Cold War during the 1950's, and Berlin was quite literally swarming with it. As it became the focal point of the Cold War, both sides of the Iron Curtain used Berlin as a place to meet with agents, but also to recruit new ones. The West needed rapid intelligence on Soviet military capabilities on the other side of the German border, much of which could be obtained through speaking to Germans coming from the East to the Western sectors. The Soviets in their turn sent many ''illegals'', intelligence officers under false identity, into West-Berlin to set up clandestine networks in West-German society and to recruit among the numerous Western military officers stationed in Germany.
The vast majority of these contacts was cheap. Both sides gave virtually anyone a small bribe for a bit of gossip or a shred of military rumor, after which they had no further purpose anymore and were discarded. The US and UK at some point had the glorious idea to dig a tunnel underneath the Soviet military sector and wiretap its telegraph cables, since Soviet officials preferred to make use of cables that could not be intercepted from the air. The Soviets had a mole inside British intelligence however, again, who ratted them out and for a while they planted disinformation until they ''coincidentally'' discovered the tunnel during construction work.
Berlin was a hotspot also because Western countries placed military SIGINT units there to eavesdrop on Warsaw Pact military communications and transmissions. They stationed technical specialists to operate intercept equipment and translators to process the material and forward the relevant bits to agencies such as the NSA in the US or GCHQ in the UK. As you mention your uncle's language proficiency, this is one thing he might have been doing in Berlin. But then I would be assuming he is an American, and was stationed by the US military (probably - like US Army or Air Force), as a linguist in West Berlin. But like I said, this is just one possibility among many. If he was an American citizen, you could probably ask for his records at the Department of Defense (I believe the NSA falls under it as well), and if they have nothing, try the CIA.
The CIA too of course employed people with language skills to operate in the field. West-German, British and American intelligence officers swarmed West-Berlin in the 1950's to recruit Germans and Russians within the Red Army, which had an enormous concentration there, and within the East-German government - especially in the background of the Berlin Crisis between 1958-1961. The CIA wanted constant updates on rumors that the Soviets might seal off East-Berlin entirely.
Based on what you have provided, it could mean a lot of things. In general, the West was actively seeking to obtain military intelligence on the Soviet bloc in Berlin, either through recruiting human sources, or through SIGINT intercept stations. The CIA and British SIS were also particularly active there to recruit Soviet intelligence officers. Wherever there is a lot of intelligence activity, counterintelligence officers will quickly join the scene to see who from the other side is willing to dance.
David E. Murphy, Sergei Kondrashev and George Bailey, Battleground Berlin: CIA vs the KGB in the Cold War (1997) - this is the best book available on intelligence activities in Berlin.