Obviously Latin was the language used by the Catholic church from these time periods, even though Latin lost use in some areas and combined with other languages/evolved to become the various languages used around Europe. But since individual languages as well as the literacy rates across Europe changed so dramatically over the centuries, it made me wonder how many people knew/were fluent in Latin throughout Europe in different regions and at different times. I assume most nobles/royalty would have had education in learning the language, but would the average person be fully able to understand it?
So the average person before the early modern period (14th-16thcentury roughly) would not have understood Latin as most people did not have access to the education necessary to have learned any of it. Entering the early modern period more people would have learned it as the increase in wealth and the diminishing influence of feudalism broke down barriers to education. There was also a renewed secular interest in classical texts, and we enter the period in which discussions of what a "true education" entailed. Latin was the language of the learned (why many publications were written in Latin. it was very odd for someone to write in vernacular). To be considered "a person of worth" so to say or a gentleman (a term used later on more commonly) you had to know how to speak, read, and write Latin. Latin continued to the be the language of intellectuals well into the 19th century but by that time it was superseded by French. Still, I wouldn't say that the average person understood Latin. Education is a system ultimately based on class and exclusivity and Latin ever since the fall of Rome was only ever able to the upper classes or the very lucky who were able to get into a monastery or afford a tutor. Hope that kinda answers your question