The (lack of a) language barrier.
When Konig is first getting his shoes shined, he's reading a Russian newspaper. He then asks the little Russian boy to translate for him. Did he ask him in German, expecting a young Russian peasant to understand? Or did he ask him in Russian? If so, why could he speak Russian but not read it? It didnt seem like he was simply hearing out loud what he already knew, it was as if he was genuinely enlightened by the information.
They should have used the same German/Russian accented English that the old 1950s WWII movies did. At least this way if they switched from a Russian accent to a German one, you could follow which language they were supposed to be speaking. Ed Harris speaks in the same voice to everyone, regardless of where they're from. Even Red October did a better job than this, and they had a Scottish guy, an Irish guy and a British guy all playing Russians!
"Oh, so that's what an invisible barrier looks like."