It's a common expression that people use when they are talking about something or someone, and then that person or thing suddenly appears. I think the reference to the devil goes back to a belief that you shouldn't talk about the devil or he'll appear.

It is A reference to someone who appears unexpectedly while being talked about. Origin This phrase is used to acknowledge the coincidence of someone arriving at a scene just at the time that they are being talked about. Clearly, nothing sinister is implied by this and it is just a jokey way of referring to the person's appearance.

"Speak of the devil (and he appears)." You use this when you're talking about someone and (coincidentally?) they arrive. Does Russian not have a similar expression? The Slovak version is "spominaj čerta (a čert za dverami)" - "think of the devil and the devil is at the door."

When you are talking to your friend about someone. And then that someone walks in

interesting, in spanish we use 'speaking about the king of rome'