Did average church-goers understand mass in Latin?

Some did, yes.

Fr. Anscar J. Chupungco, in his What, Then, Is Liturgy?: Musings and Memoir p. 3 (quoted here), gives an anecdote of how one of his parishioners reacted to the post-Vatican II change of the priest's prayer when distributing Communion from the traditional "Corpus Domini nostri Iesu Christi custodiat animam tuam in vitam æternam. Amen." ("May the Body of Our Lord Jesus Christ preserve your soul unto life everlasting. Amen.") in Latin to the terse vernacular "The Body of Christ":

In the Church of Sant'Anselmo an elderly lady corrected me as I was offering her Holy Communion: "Non dicitur 'Il corpo di Cristo,' sed 'Corpus Christi'!" (In perfect Latin she bade me say "The Body of Christ" in Latin, not in Italian.)

The lady said: "One does not say 'The body of Christ' but 'Corpus Christi…'!"

No, most did not understand Latin.

My mother grew up in a very strict RC family in the 1920's. The background is lower middle class. Her brothers had a good education. Some became priests (obviously they learned Latin), most of her other brothers learned some Latin in school.

She told me they had to learn and memorize the phrases they spoke in church. Only those particular words and phrases they knew. My grandfather would test their knowledge to that extend. (There was hell to pay if they made mistakes!)

From my mother's family of 14, 3 of her brothers became priest, 3 of her sisters nun. That was well over the 'quota' the church found reasonable. So much so that the parish priest visited my grandfather and asked him not to send my mother to become a nun. He had done his duty for the church already...

This is, I know, anecdotal evidence. However, consider that my family wasn't uneducated, we're talking about the 1920's with reasonable to good education. Even then most laymen did not know Latin, apart from the phrases used during mass.

Which would be a lot less in periods before, when education wasn't as widespread. I don't speak Latin, but as far as I know there is a difference between 'church' Latin and 'real' Latin, as taught in better schools.

The question was:

Did average church-goers understand mass in Latin?

Not if highly educated parishioners could understand mass in Latin. Latin was until and beyond the enlightenment age the language of the educated class. Desiderius Erasmus for example was Dutch. He published mainly in Latin. He corresponded in Latin. So did most if not all scholars in the period. These men were not the average churchgoers.

It varied in degree depending on the period and the location.

As explained in this podcast on the emergence of Romance languages from Patrick Wyman's Tides of History, the line between Latin and the variety of secular Romance languages that appeared during the Middle Ages was extremely blurry for several centuries.

Essentially, for several centuries people would continue to read and write in Latin, but with they'd read it with an increasingly local pronunciation. This process was already occurring by the time Rome fell, and it took several more centuries (around the 10th century if memory serves me well) for secular Romance languages to have diverged enough in pronunciation that they began to appear in written form. Given this, it stands to reason that peasants would understand the bible passages being read to them provided their priest would read it with their accent.

After that transition occurred, Romance languages slowly but surely continued to diverge from Latin. Still, the two aren't entirely unintelligible, including today. This is not to say that a modern French or Spaniard would be able to take a Latin text and instantly make sense of it, of course. Just that knowing one makes the other intelligible to some degree by virtue of the amount of familiar words.

(For other secular language I've frankly no idea, but I'd wager the answer is mostly no.)

Literate people can carry dual-language missals. My father did first communion in the old rite. Even as a child/young teen, he could follow the gestures, context and more or less the Latin sound/text, and read the Portuguese text.

After some years, it was really easy. I have been at masses in Latin and other languages myself. After all, it is easy, by the gestures, context and sequence of events to know more or less on which step the Mass is. For example, you lost track because you are in a sleepy day, but the chalice is on the altar and you have not heard the bells yet: it is in the beginning of the Eucharistic Prayer, just open in the right page and hear. Even if you do not understand much, a gesture or something else will tell you the exact place in due time. The old rite only had one Eucharistic Prayer, directly from the first centuries, so it did not even had the varied texts we use today.

It does not take a PhD in foreign languages to do that. The extra mental effort even helps to keep the attention.

Obviously, many people can not do even that. But, if they know the general steps of the Mass, the meaning of gestures, and they at least know the general meaning of the corresponding text in their language, it does not matter if they do not understand or follow the words: they know what is happening. They can utter other prayers silently and/or just mentally follow the meaning of the general steps of the mass. For example: the priest is beating his chest, I know that it is the confession of sins, I know the meaning of the text, then I think about my own sins; then the priest is receiving the bread and wine into the altar, and elevating them (not the consecration, before that): It is offering time, I know the general meaning of the texts, and I offer my week's sorrows in my mental prayer, as the community offers the bread and wine; After that, it comes the preface of the day: the time of thinking about the feast of the day. Then, the consecration is clearly visible and heard – the altar server rings his bells. Jesus has come to us!

This is what I have done for a few weeks in Germany with no written missal. I barely understood a word besides Vater, Sohn, und der Heilige Geist, but I was following the mass by the gestures and the sequence of events. I clearly understand how simple illiterate people (illiterate as in not reading in their native language) could hear the mass in Latin, pray the rosary at the same time, and still profit a lot – this was very common in Brazil.

Obviously the Latin mass is not as easy for non-latin and even more to non-indo-european languages. This is way there are the exceptions that you cited.

An interesting info that you may have missed: not even priests were required to speak Latin fluently or well. They only had to know enough to understand the text of the missal by themselves. This is why having Chinese priests using the Latin missal was possible (yet still very difficult).

Another take: The Latin problem was not relevant in the middle ages. During the early middle ages, Latin was still spoken and the romance languages were not as developed.

Moreover, even after the languages diverged from Latin (Late Middle Ages), they had not converged into a coherent standard. Suppose that in 1300 (before Chaucer or Samuel's dictionary) the English Church decided to use English: which English dialect? There was no standard written form. No widely accepted dictionary. If the Church had chosen one, it might look as preferential treatment, as a political issue - and even then, some scholar would have to decide uncertain details about orthography and grammar in order to be able to write a missal. Would you have a translation to each dialect?

The problem is even worse for German - some dialects were not even written down. In France: langue d'oc, or langue d'oil? And the Bretons and Basques? Don't even start thinking on Spain or Italy before Cervantes or Dante, their books really made messy dialect collections look somewhat close to one language.

Every translation requires a corresponding authority, and raises many issues. They have a missal, soon they have translation issues, then they want a special saint calendar for their local saints, then they want to change the rites a little bit. Then disputes arise and need a judge familiar to their local rites. It is a lot of work - and the western church do not have patriarchs as the eastern rites. The current national bishops conferences were created to deal with that, and they created other issues - a long story.

The exact frontiers of the current national bishop conferences are not a pressing issue because the borders have not changed much, it has been generally a peaceful century (luckily). But how exactly would you group dioceses in middle ages into different translation conferences, without ruffling royal feathers?

Edit about the silent canon: My first example about the boy waking up specifically during the Canon is not a good example, as it was silent. But the waking-up boy would still be able to recognize that the mass is in the canon, due to the silence, the presence of the chalice in the altar, and the fact that the bells have not sang yet.

With the silent canon, it makes even more sense for illiterate people to say their own prayers, such as a rosary. Even if they barely knew the meaning of what is happening, they would know it is the most important part of the mass. And literate people who have or know the text could follow the different gestures during the canon.

Given my experience in Germany (my time as a illiterate peasant at mass) it really would not matter if the canon was silent or ad Orientem (priest facing the altar) - I could only follow gestures anyway.

It can not be that hard, they did that for 2000 years...

And yes, the reformation is clearly linked to the emergence of national states and unified languages. After all, the reformed prince becomes the boss of both secular and spiritual swords with their incomes... A step in centralization of power. And the problem of imposing a national protestant rite implies imposing a national language: another step on unifying country and language under the boss and over the minorities.

In the Middle Ages they did not have the unified nations and the unified languages we have today, so the concept of a "national Mass translation" would be hard to even define.