"He speaks behind his back."
Translation:Lui parla alle sue spalle.
I'm not a native speaker, but a google for "parla dietro la sua schiena" gave me only one hit, in a context where it was clear that a physical location was meant. I suspect that with sufficient context it could still be understood as "talking behind his back" in the English sense, but would probably sound quite unnatural.
Marialramendy: While that's true, it's an idiom and idioms don't / usually don't translate the same way from one language to another. In this case English says "back", but Italian says "shoulders." Another example: We say, "I've had it up to my neck" while Italian says "I've had it up to my hair" . Another: English: "Don't lose your patience!" Italian: "Don't lose your stirrups!" So you can't expect 1:1 equivalence when dealing with idioms or common sayings.