German is not thought to be a particularly difficult language for English speakers. The grammar isn't too far from what we're used to, the alphabet is familiar, and aside from the notorious "ch" and "ü", the pronunciation isn't very challenging. One common point of confusion, however, is the usage of "ie" vs. "ei." Whereas "ie" is usually pronounced in English like "eye," (e.g. pie, die, lie, etc.) and "ei" like the long e, (e.g. weird, receive, etc.) in German it is the exact opposite. I've got a friend who refers to his "beer stiens"; it's actually beer stein, from the German word for "stone." And I still recall observing a speech tournament in high school where one of the finalists in international extemporaneous speaking kept quoting from some publication called "Dye Zeet." It was several minutes before a German classmate of mine realized she was referring to the national weekly newspaper Die Zeit!
An especially confusing one for musicians is the piece "Liebesleid" by violinist and composer Fritz Kreisler. It means "Love's Sorrow," from "Liebe," love, and "Leid," sorrow. But there is a German word far more familiar to musicians: "Lied," or song. And certainly "Liebeslied", or love song, would be a reasonable name for a piece. It's no wonder that those unfamiliar with Kreisler's work often call it by the wrong name.