When did they start calling priests “father?”
From the Online Etymology Dictionary
The romance languages all use the same term for priest and father. The Pope is Il Papa in Italian.
Father is derived from a common proto-Indo-European word pa-
A list of Indo-European terms for father show a series of cognates:
Priest itself is from the Latin for elder, a similar connotation:
So if I understand all of that, the real question is, in English, why and when did we stop using “father” as a job description for a priest?
In other words, in English we’ve retained it as a form of address only. We say, “Joe is a priest”, and address him as “Father Joe”.
Whereas in Spanish, Jose is a padre, and we address him as Padre Jose. Does the separation have anything to do with the English Reformation?
I’m not sure that your Spanish example is correct. When I was studying Spanish (lo, these many generations ago), we used sacerdote and cura to translate the word priest. The U.S. (and British?) military has a habit of using the Spanish padre as descriptive noun as well as a title, but I am not sure that that usage holds among speakers of Spanish.
As to the dating of either YPOD’s question or that of Freddy the Pig, I don’t actually have the answers and I have not yet found a clear reference.
We’re not alone Freddy the Pig in French the job is “prêtre” but he is addressed as “mon père”.
I’ve never heard any of my Italian friends talk about a padre! They also have a “sacerdote” but the commonest word is “prete” altho’ I have also heard “predicce” (I’m unsure of the spelling, it could be a Veneto dialect word) which is literally a "preacher.