"Word choice in a document like that is so important, so I have to believe that they are trying to tell us something specific here; unfortunately, without them defining their language, it's impossible to know what that something is!"

Totally agree, abbysmum. I speak fluent Spanish and very often translate for non-Spanish speakers. One NEVER translates literally. You have to translate so that it makes sense to the non-Spanish speaker. Many, many words and idioms are not able to be translated literally. They make absolutely no sense in another language.

With that in mind, is the GIRM a literal translation from the Latin? Are there Latin words that should not have been translated literally, but should have been modified so that the meaning is fully understood in English?

Well said. My first language is English, but I am fluent in French and was educated in French. One of the things I learned in high school was that idioms don't translate well. That's a big part of "lost in translation", because equivalent idioms don't always exist (with connotative expression being the other large factor - something closely linked to idiomatic expression). And even when equivalent idioms exist, they rarely use the same words or pictures (although they might be expressing the same idea).

I don't know Latin well enough, but I've always assumed because Latin was a dead language, it lacks idioms. Is that a correct assumption?

There are idioms and also idiomatic registers of usage.

For example, one of the things I was taught in Spanish is the different in idiomatic register between profanities and slangs of different regional Spanish usages versus their seeming cognates in English regional usages. (A vital thing to understand in conversation, and not to be left far too advanced in learning.) Quebecois French is masterclass of examples on this terrain.

Bottom line: when it comes to languages, usages, and translations, always assume there the possibility of missing subtext. It's not plug-and-play, so if you're a person uncomfortable with missing subtext...be very patient. I spent my childhood in a large family observing how my parents - who shared values deeply - had very different usages and registers and thereby often misunderstood each other and the mix that percolated among their many children. My entire professional work was based on that youthful observation combined with my immersion in a foreign language at a young age, because I realized the inherent plasticity of language and meaning (intended and received) was a nearly universal human pattern. (One of my siblings is significantly neuro-divergent and she could never quite catch onto all of the humor of the rest of the family and it was isolating for her.) My reaction to "but language is fixed in meaning" as a way to avoid messiness is: Sure, Jan.