Isn't that what I said, GUEST?   

Just saying it wouldn't be spelled "Que sera sera" in Italian. : )

The French would probably say "Ce que sera sera" in stead of "Que sera sera," but I'm not really bothered by a song phrase or title not being a common linguistic idiom.   I'd say an awful lot of song titles and hook lines are not common idioms (unless they become so because of the song).