To answer your last question, yes. There were indults granting vernacular hymnody at High Mass in Germanic lands, vernacular propers on Indian reservations in the U.S., the Tridentine Mass said in Old Church Slavonic instead of Latin, etc. These things mostly came from long-established cultural practices though as opposed to stemming from excessive optionality as one finds in the current missal.

There are approved local adaptations of the GIRM, set forth in each local Missal edition.

In practice so long as they are not there long enough to be quasi-domiciled, or do not say a public Mass for anyone except pilgrims, then they don’t have to follow it. I have an American friend who lived in France. Obviously, when he said public Mass in French, he needed to follow the proper parts for every Sunday that French-speaking areas had for the Eucharistic prayer as an adaptation (at least until they got their new missal). But they aren’t in the Latin, so his Masses in Latin didn’t use them; ironically, the problem is that priests less familiar with the Latin NO didn’t know this.

The calendar is another place where there are obvious conflicts.

The preconciliar rubrics specify what calendar to follow for seculars and religious who are traveling and such, but that only made sense in the period when someone in the church building itself could point it out to you, even as the rubrics became messy, to the point of each diocese not only having a different ordo but with disagreement on the things in common (unfortunately not unusual from 1912 to 1954) and where almost everyone had the same basic missal. Now, even if you could theoretically just use the common, it feels a bit silly to follow the proper calendar in another vernacular — even if you don’t wind up following your own national calendar since you’re abroad.

Unity is not uniformity. I think that there are some local adaptations (what we call them now) and indults or whatever (previously, these decrees were all separate from the rubrics themselves) that need to be suppressed or which are unwise but tolerable. But the principle is the same.

In the context of the 1962 Missal, I think it would be fantastic to have some kind of study of local adaptations (whether by custom or decree) by country. There clearly were some interesting local differences.

I'd also be interested to know to what extent post World War II diaspora communities were permitted to retain 'country of origin' customs and laws. I can't imagine, for instance, the hundreds of European refugees temporarily resettled in parts of Africa, during and after World War II, following the local calendar so 'religiously' as to give up their proper national feasts.

After that, it would be interesting to get a binding ruling as to what can be used where.

I am thinking, in particular, of whether the Weller Rituale could be used throughout the Anglosphere.

@MatthewRoth that is not exactly true. In the first stages of Sacrosanctum Concilium, there was some approval for parts of the Weller ritual - probably since it was the only complete one extant at the time. See the NCCB decree of April 1964.

Re: the calendar - nowadays, in my (admittedly limited) experience, most priests are woefully ignorant about the liturgical calendar, as are many seminary liturgy instructors. I doubt you would even find instruction in a seminary course or practicum on what is a priest's proper calendar. The religious may have some rudimentary knowledge, but even they are not instructed.

Part of the problem is also that most of the information on proper calendars that was laid out in the Rubriciae Generales of 1962 was moved to a separate instruction and then promptly forgotten. And given that many priests are less particular about getting parts of the Mass correct, and being bound under pain of sin to do X and Y......proper calendars don't get a lot of airtime.

Yes. But the point is the translation which blows. And it’s 1964 permissions whereas we are bound to 1962.

@MatthewRoth Fortunately we have this, https://archive.org/details/ExcerptaRitRomAngliaeCambriae/page/n3/mode/2up