Salvete amici et amicae!
Nescio dicere latine “Speaking of…” etiam “Speak of the Devil!”
Gratias vobis ago, amici et amicae!
Valete
“Speaking of…” = “[sic] in dicendo”
“Speak of the Devil” = “Lupus in fabulâ” (Traupman)
“Lupus in fabulâ” is kind of adorable.
Does anyone know the origin of the saying? Is it even possible to find out the origin of the saying if we lack someone native from the culture? 
This seems to be it. Specifically:
You can find the saying > lupus in fabula > in Terence and in Cicero, and also in Plautus, in the form > lupus in sermone> , “the wolf in the talk.”
As for where they got it from or why the expression arose, I don’t know…
Salve columbula
- Donatus quoted in // Quod hîc citatum est: Thomas Williams, “The Vestiges of a Roman Nursery Rhyme at Donatus in Ter. “Adel”. 537” , Mnemosyne, Fourth Series, Vol. 23, Fasc. 1 (1970), pp. 62-67
I translate as follows the passage from Donatus // Ille locus apud Donatum sic à me vertitur:
“The wolf in the story” is an indiction of silence in this saying [in Terence’s > Adelphoe> ] and moreover of such a kind of silence that he [the speaker] should right in a word or in a syllable cease to talk, because we say that men have seen a wolf who suddenly become silent; insofar as it happened to them that a wolf just earlier had seen them, that with the recognition of their situation even their words and voices failed them. For thus [says] Theocritus (Idylls. XIV 22) “‘Do you say nothing?’ says one, and joking, ‘Have you met a wolf?’” and Vergilius (> Ecloga nova, versus quinquagesimus tertius et quartus> ) “and even his voice has deserted Moeris, the wolves having seen Moeris first”. Others reckon [it’s] from the stories of nursing nannies to frighten boys for their carryings on by the coming of a wolf bit by bit from Capua right up to the bedroom threshold; on the other hand, it’s not the case what’s said that the wolf had come from the “Upbringing of Remus and Romulus” of Naevius’s play, when it used to be performed on the stage.
Nowadays the saying is an indication of coincidence; then it was one of nervousness or fear, albeit perhaps also with a deliberate naif or funny undertone (just the same as the one which today dominates). The wolf represents one who stalks a defenceless prey.
Nunc ut signum concursûs passivi proverbium dicitur; tunc signum metûs seu terroris, etsi fortè etiam consultò coloris ingenui vel jocosi (et eiusdem coloris qui nobis diebus dominat). Lupus demonstrat eum qui praedam indefensam captat.