[Don José]

I have remembered a film (The barefoot Contessa) about an Italian family that used that expression in Italian, and have looked in Google. According to the Wikipidedia, the origin of this mistake was an Italian grammar mistake in that film (modern standard Italian, they say):

There has been some confusion about the identity of the language in the song's title and lyrics. The words are Spanish, but the phrase is ungrammatical in Spanish. (In grammatical Spanish a roughly equivalent idea can be expressed as "Lo que sea será."[5]) Composer Jay Livingston had seen the 1954 film The Barefoot Contessa, in which an Italian family has the motto "Che sarà sarà" carved in stone at their ancestral castle. He immediately wrote it down as a possible song title, and he and lyricist Ray Evans later respelled it in Spanish "because there are so many Spanish-speaking people in the world." [6][7] Early in their career, Evans and Livingston had worked together as musicians on cruise ships to the Caribbean and South America. No other language was involved in their coining of the phrase.
"Que sera sera" (with this evidently Spanish-based spelling, but no accent marks) appears as the motto on an English family coat of arms described in William Bartlett's 1865 history of the parish of Wimbledon.[8] But no variant of "Que será será" appears in any of the books in Spanish scanned by the Google Books project, with publication dates from 1500 to 2008.
Although "Che sarà sarà" is also ungrammatical in modern standard Italian (where the idea could be rendered "Quel che sarà sarà"), it does appear in an English context over 400 years ago, in Christopher Marlowe's play Doctor Faustus (Act 1, Scene 1), whose text contains the line "Che sera, sera / What will be, shall be"). The Italian version of the saying (spelled "Che sara sara") also has served as the heraldic motto of the Dukes of Bedford (England) since at least as early as 1749.[9] It is not known whether Joseph L. Mankiewicz, the screenwriter and director of The Barefoot Contessa, was aware of this use of the slogan.

http://en.wikipedia.org/wiki/Que_Ser...Be,_Will_Be%29