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).
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.