No big deal. He probably grew up speaking Yiddish and learning in Lashon Kodesh (meaning a mixture of Hebrew and Aramaic). That means he was already, by bar mitzvah, bilingual in both a Semitic (Lashon Kodesh) and an Aryan/Indo-European language (Yiddish). That makes it fairly easy to pick up languages. Also the local dialect was a slavic language, so picking up a new slavic language would be a “piece of cake”. Picking up a more distantly related Aryan language, such as Iranian or Greek might have taken a bit longer, and a language from an unrelated family (Chinese, Mohawk, etc.) would have been harder. There are tremendous economies of scale in learning languages. The reasons Americans are so dumb at language has to do with laziness and arrogance since we are spoiled by the fact that English is the de facto “lingua franca” among the goyim – in the 18th century educated Americans typically knew several languages, and throughout history frum Jews have always been excellent in learning languages.