Oh, hey, a question that I'm qualified to answer. I'm mainly going to talk about pre-Reformation, since I'm a medievalist. I’m also going to cover a fairly narrow window, to wit, 1215 to 1517. 1215, because the Fourth Lateran Council marks the standardization of a lot of the Catholic Church’s doctrine and practice that it would keep down to the present day, and 1517 because once the Reformation kicks off things get a bit weird and we get outside of my scholarly wheelehouse.

In medieval Catholicism, the language of the liturgy, of theology, and of canon law was Latin. There were a few reasons for this: one was a simple conservatism that you often see in religious rituals and the like. But then there was another one as well: Latin was an international language. In an age before vernacular languages had standardized grammar and in an age before mass media, even vernaculars (e.g., English, French, Castilian, Occitan, etc. etc.) would themselves be divided into lots of different dialects that would be pushing at the edges of mutual intelligibility. But Latin had standardized spelling and grammar with a in international cadre of Latin speakers. This means that (for example) a book written in the Scottish lowlands can be read by someone in Dalmatia.

But of course, Latin literacy -- and after all, school was all in Latin -- only reached five to at most ten percent of the population near the end of the Middle Ages. How do we deal with a Christian population that doesn’t really know Latin? Are we going to catechize them in Latin, preach at them in Latin, and have them say their confession in Latin?

No.

So let’s start with basic catechesis. From the twelfth century on, we know that the Church as an institution at least had the ambition (if not always carried out in practice) of making sure that everyone had at least a baseline level of religious instruction. Let’s have a look at how this applied to the parish priest. Bishops would often gather the clergy of their dioceses in synods where they’d issue legislation that priest’s would be required to keep on hand. Lots of the requirements are the “nuts and bolts” of day to day church life, e.g., don’t let people keep livestock in the parish church, don’t hold a kegger in the church for fundraising, make sure the priest’s vestments and church fabric are in good repair, etc. (These statutes are, by the by, a wonderful source of social history.) But they also very frequently require that laypeople know the Apostles’ Creed, the Hail Mary, and the Lord’s Prayer. Now then, for the Ave Maria and Paternoster, it’s fine to be able to say the Latin, since it’s really more a matter of process than cognition. But with respect to the Creed, lots of bishops’ statutes say that laypeople should know the Creed, “at least in the mother tongue,” saltem in lingua materna.

Moreover, these same statutes will sometimes mandate that priests teach the children of a parish (although more often the requirements would fall on the parents and godparents to give their children basic catechesis). Sometimes synods would give quick explanations of the basic doctrines that Christians should know, and sometimes bishops would append little booklets that had the basic guide to administering a confession and carrying out catechesis, breaking down the Christian religion into its articles of faith.

So let’s talk about confession. Confession goes a bit far afield, but in England (which is where I’m the most comfortable, but you see similar in late medieval France and Germany), you’ll sometimes have formularies for someone making a confession that will appear in a vernacular, and oftentimes as well, those little booklets on confession and catechesis would have explanatory glosses or marginal notes in a vernacular. These marginal notes indicate that the priests might be reading it in Latin, but they’re explaining the material in English, French, German, etc.

Okay, I’ve talked about catechesis and (a bit) about confession. Now, let’s talk about preaching…