"Speak of the devil" is the short form of the idiom "Speak of the devil and he doth appear". It is used when an object of discussion unexpectedly becomes present during the conversation. For example, if Alice and Bob start discussing Charlie while he isn't in the room, and Charlie walks into the room, Alice or Bob might say, "Speak of the devil!"
Лёгок на помине. Нашла-таки эквивалент на русском.