Let me get this clear. Are you claiming that the ‘average person’ could understand some at least of the Latin liturgy even if he or she could not read or write Latin?
If you are saying that, I agree with you to an extent. I believe that the sense of the liturgy, that is, the understanding of the offering, the understanding of certain words such as “Dominus/i” for Lord, or “peccata/peccatoribus” for sin, 'Oremus" for 'let us pray" was understood, not because the people were using Latin in their common speech, but because through decades and centuries of Catholic Mass and Catholic understanding passed on from priest to people, and family to family, made it so.
However, that does not mean that the average person could for example ‘hear’ the Roman canon word for word and ‘translate’ the Latin into English, French, or German. But they didn’t ‘have to’. They understood that the words were sacred, that it was a prayer of offering of a sacrifice, that the postures, vestments, actions, and words, even if imperfectly heard or understood, were understood in their hearts through all those actions.