Fascinating graph. "The phrase actually comes from a Medieval Latin proverb, “Graecum est; non potest legi,” meaning “It is Greek; it cannot be read.” From there, the phrase filtered into many European languages. Today, English, Spanish, Polish, Norwegian and Swedish all use Greek as a metaphor for incomprehensibility.
Mark Liberman, a professor linguistics at the University of Pennsylvania, started wondering what the equivalent was in different languages. Drawing on a variety of sources, including Wikipedia, Omniglot and user comments, he created the graph above, which shows the language that other languages use to describe things that are hard or impossible to understand."