Why not? Most of the magi coming on board were descendants of the Cult of Mercury, and had nothing to do with Hellenistic occult traditions.
I don't think I'm going to change this. The English name is the Code of Hermes, has always been the Code of Hermes, and is still the Code of Hermes in ArM5 (see page 13). The current layout makes this unclear, so I'll have the title of the insert changed. I think Lex Hermetis and Ordo Hermetis would be perfectly good alternative names, but I don't see a strong reason to change the ones I've been using for thirty years. (Although that's a good concept for a Tytalus. "It should be Ordo Hermetis! We are not an order of a statue!")
And as for what Bonisagus was thinking — well, anyone can speculate, but I can decide.
For what it’s worth, I’m both an Ars fan AND a Latin teacher. Iusiurandum is the best word for an oath. See this older text of the Hippocratic Oath (it’s from the 16th century) that uses the word with the genitive of Hippocrates name in front.
As to what to use for Hermes/Hermetic, most Latinists use Mercurius but with the occult tradition I could see Bonisagus wanting to use Hermes, though most instances of Herma refer to herm statues.
First and briefly: as written, the Code or Oath of Hermes is a constitution cast into an oath of individuals. So it can't be just a law, which implies a prexistent legislative body deliberating, deciding and imposing it.
"Code" is ambiguous in English. If you look for "rule", "statute" or "duty", you again find "ius" - e. g. here:
"Leges", the plural, still implies the Code of Hermes to be imposed laws. It might come closer to some idea of a "code" as a collection of laws - like famous French Code Civil - but would also be a an exaggeration in front of less than half a page of text.
"Corpus" - like in Corpus Iuris Civilis - may fit an idea of a voluminous "code", but not of half a sheet. So certainly "Ius Hermetis" is more fitting than "Corpus Iuris Hermetis" or such.
Other, still a lot weaker translations are:
"statutum", past participle of statuo, for "statute",
"officium" for "duty", "office", "function".
"Praeceptum", past participle of praecipio, for "rule" might be fitting for case law by a Tribunal, so "Praecepta Tribunalis Rheni" might be the Peripheral Code of the Rhine Tribunal.
B.A frontier town of Bœotia, over against Eubœa, [Liv. 35, 50, 9](No document found. 35.50.9&lang=original).
So in classical Latin "Hermes, hermae" is "Hermes pillar". The name of the god in classical Latin is Mercurius - and even Hermes Trismegistos in classical Latin turns out to be Mercurius ter Maximus
We hence have to look for uses of Hermes in later Latin - like the title of the "Liber Hermetis".
This is the only one I find easy to map from English to Latin so far: Lex (law).
In particular, it means simultaneously a single informal rule (like the lex talionis), a small, specific set of rules (a single law, like the Oath of Hermes), a body of laws (like the Lex Salica), and the law (which wins when you fight it ) Dura lex, sed lex!
Codex is a possible alternative (and in fact it's where Code comes from), but it provides more an idea of "written compilation of legal stuff" rather than "something you have to obey to". So the basic Oath is too small for Codex, and the Peripheral Code too dispersed: the idea behind Codex is that it's stuff collected in a single place - specifically a book or a set of books. In fact that's the big innovation of the Justinian code: a set of books that gathered together and reorganized according to common abstract principles a pre-existing and disparate set of laws and rulings.
I would avoid Ius. The basic idea of Ius (the same root as Justice) is not one of duties, but one of rights (including rights that you must accord to others, so that's where the idea of ius as law comes from - think ius primae noctis). For those who speak French or Italian in the forum, it perfectly translates the notion of droit/diritto in its twin sense of "right" and "jurisprudence". The Ius Bonisagi would make one immediately think of the legal right of a Bonisagus mage to claim someone else's apprentice. But the bulk of the Oath and of the Code seem to be "thou shalt do neither this nor that nor anything else that might cause trouble!". So (dura!) lex, not ius.
However, there is one thing in which I agree with OneShot (a rare occurrence!): Lex Hermetica (or Lex Hermetis) is far better than Lex Hermae, because when I read Lex Hermae I immediately think of "the law of the herm" i.e. some specific law about one of the herms, the statues of Hermes/Mercury often used as markers - say, something that gives someone the right to place the first one in a particular area. "Hermeticus/a/um" is indeed a bit ... hermetic, but I agree with OneShot it's what a person who spoke both Latin and Greek would go for when making up an adjective referring to stuff of the god Hermes called by his Greek name (which definitely made it sound more exotic and mysterious, Romans - particular in the imperial age - loved this sort of stuff).
Note that Lex Hermetis is a better translation of Code of Hermes, whereas Lex Hermetica is a better translation of Hermetic Code. I tend to find Code of Hermes a bit ... off compared to Hermetic Code, in that it seems that it's the Code of/from the god Hermes himself, rather than that of the Order of Hermes (so I prefer Lex Hermetica) but that might be just me.
I think I will be sticking with my original choices. They are sensible candidates for the translation, so there is no strong reason to change them. (Yes, you've only just seen them, but I've been using them for thirty years — and I'm the one making the decision.)
If you don't agree, all is not lost. Thanks to the Open License you will be able to do your own translation and sell it! OK, strictly speaking that's a stretch goal, but the stretch goal that opens the Code of Hermes is low enough that I will be profoundly shocked if we don't make it.
But the Codex Theodosianus and Codex Iustinianus were already huge compilations of laws, and the latter became an important part of the Corpus Iuris Civilis. So they can't serve as examples for naming the Oath or Code of Hermes.
A Codex Hermeticus might, however, be a collection of all still valid decisions of the Grand Tribunals - should some magi compile such some time.
So these are your choices then. You could, however. also just drop the Latin Oath of Hermes from the Definitive Edition. Is it really a necessary part of it?
The Oath of Hermes establishes a society of magi and defines their constitution, thereby establishing a "ius magorum", not just "leges". And as an oath proper it is by itself also "ius", not just "lex".
I just want to say that dubious Latin words and pronunciation is part of the fine history of Ars Magica going back to its earliest days and the title of the game.
When in doubt, and when there are competing options, my inclination is to rely on what makes more sense to modern eyes at first glance, even if it would seem weird or stilted to an ancient or medieval speaker of Latin. I'm also happy to wave hands about how the Order has developed its own peculiar argot and usages over the centuries that would seem a bit alien to the average monk hunched with pen over parchment in the scriptorium of Cîteaux.
It's not been suggested yet, but I tried figuring this out a few years back and ended up with "Juramentum Hermeticumen" <- basically straight up Oath of Hermes (but my latin is very hobbyist)
Juramentum is a Latin word that means oath. It is a promise that someone makes, usually in a legal setting, to tell the truth or fulfill a duty.
I believe Juramentum actually got discussed, and it refers to the oath making process. Jus Juramentum is the thing being sworn, and is usually shortened to Jus (or Ius).
Well we know that the Order has their own usage variation of Latin, as shown by the Specialty 'Hermetic usage' (compared to 'academic usage' and 'Church usage'). So members of the Order using variations of Latin which seem "wrong" could easily be justified. This would match the actual game mechanics difference between reading something with 'academic usage' and 'Hermetic usage'.
In fact it might be an interesting little project to catalog these 'Hermetic usage' differences in published books, showing how the Order's version of Latin has drifted and changed slightly from other versions of the time.
) Dura lex, sed lex!