To say that Latin, French and English were in use in England in mediaeval to early modern times is not to imply that there were no other languages. There was, for example, Hebrew, as in the Alexander saga, where, opening up the modern translation side at a random page, we read:
“Afterwards Alexander sent the following letter to all the provinces of the kindom of Persia: “Alexander, son of the god Ammon and son of Queen Olympias, to all the princes and peoples in all the provinces of the kingdom of Persia …” ”
— Israel Kazis, The Book of the Gests of Alexander of Macedon: A mediaeval Hebrew version of the Alexander Romance by Immanuel Ben Jacob Boneils, (1962) [Print-on-Demand* 2016] (Sefer Toledot Alexandros ha-Makdoni), s 74 (p 113).
Aside from the reference to ram-horned deity, in form and structure, this letter’s greeting is just like a Common Law writ, with the mediaeval king of England writing to the sheriff of a county and charging them with a duty in a legal matter. And the same wording carried through centuries.
Diplomatic letters between ancient kingdoms thousands of years earlier also followed similar patterns, or precedents.
Copying an earlier example as a template makes good sense. Over time, though, archaic language features (words, syntax, sentence structures) barnacle themselves into the process and then conjectures naturally arise as to why the language has to be archaic.
It “besots lawyers and nonlawyers alike” — Bryan A Garner, Garner’s Dictionary of Legal Usage, 3rd edition, (2011) [Oxford University Press, 2011], “Myth of Precision” (p 596).
with Garner assigning it to ‘the myth of precision’ (understandably enough, since a straw man is needed as antagonist in his quest for Plain Language).
The technical term (in some jurisdictions) for an earlier exemplar letter/deed/contract/lease/writ in general/etc is ‘precedent’.
“In England, Australia and Canada, lawyers use precedent to refer to a legal form. American lawyers speak of a form, while Scots lawyers speak of a style.” —Garner’s, “Precedent D.” (p 696).
“in Scots law, a model form or precedent of a deed or pleading” — “Style 3” (p 849).
* As an aside, not infrequently, self-printed and print-on-demand works start on the wrong page (2, instead of 1), thus throwing everything on what would have been odd pages onto even pages, and vice versa. Whether readers or the person pressing the Print button notice is another question.