It depends how far you go back. Certainly in 16th century Europe I think we can see the change from prayers in Latin to prayers in the vernacular, mainly for the protestants, but for the Catholics I think the change came a bit later. One sees references even in literature where a French person (prior to the Revolution the late 18th century) speaks of having said an “Ave” or a “Pater” which would indicate that the prayer was said in Latin, not in French; ditto references to the early Spanish settlers of California, and Florida. Certainly prior to the 16th century people knew their Aves and Pater Nosters, in fact well enough that the rosary (in Latin) became extremely popular for the laity to be able to pray ‘along with’ the priests, brothers, and religious sisters/nuns who also recited their prayers/Divine Office etc. in Latin.
I think many people have noted that, with the Romance languages especially, there are so many cognates of the Latin words that even the ‘illiterate peasantry’ could be capable of easily understanding those prayers. Not to mention that prior to the 20th century, most every European, even said ‘illiterate peasant’ had a smattering of vocabulary in more than his or her native tongue. If you lived for example in one of the English port cities, you would know enough French, and probably some Dutch, and possibly some words of Spanish or Italian, simply by living there and either engaging in trade, or service, to the many people of that nationality who utilized the services of your town. Certainly you probably could not be considered in any way ‘fluent’, but you could manage with probably at least a couple of hundred words and some ‘gestures’ to get your ideas across to each other. And of course, if you were engaged in a business which routinely dealt with say a lot of French merchants, you would know a lot more, in order to run your business successfully.
And of course if you were a middle class or wealthier individual, even your daughters would be ‘taught’ French and Italian as a matter of course, and your sons would attend colleges and be taught Latin and Greek --and that was still the case well into the 20th century.
So I think, although I’d have to do some research to get real ‘facts and figures’ that for the majority of the English-speaking countries, the vernacular prayers for Protestants came into play (from the Latin) around the early 17th century; the other European Protestants/countries about the same time, but the main Catholic countries/individuals more like the late 18th century, and the areas that became Germany and Italy probably somewhere between those two points, with areas/individuals who may have kept up the tradition of Latin well into the 19th and even 20th centuries. . .and of course, some Catholics today who are ‘reviving’ the use of Latin prayers at home.
