I think it depends on just what you think the analogy is. I was thinking of the old (and to some extent present-day) predilection for using Latin for mottos.
In what way do you see the analogy?
Old High Vilani is analogous with Latin in being a old language, but I don't think it is analogous in being used in the same way -- as an active language that allows scholars from one end of Christendom/The Imperium to the other to speak with each other.
I would guess that realistically there would not be an exact analogy (there is no reason that the Third Imperium should be an exact parallel to Old Europe).
But Old High Vilani would be the language of culture that pervaded the First Imperium, and both it and Old Common Vilani would have remained extant throughout much of the Old Imperium territories throughout the Rule of Man. The Terrans would have brought with them Old Anglic and made it both the language of government during the RoM, as well as the common language as spoken in the Terran Confederation and the rimward extents of the Vilani Imperium (as those areas had been held by the Terrans for quite some time before the final fall of the Ziru Sirka).
So having thought about it some more, if I were to make an analogy to the Ancient World on Earth, Old High Vilani would fill a role somewhat like Greek did during the Roman Empire: a language of culture and learning that covered most of the Hellenized World, which was then later subjected to Rome, just as the Vilani were subjected to the RoM. Old Anglic is therefore somewhat analogous to the Latin of the Roman Empire: the Language of government and the "common language" of the time, that was superimposed (and more prevalent in some regions, and less in others).
But just as with Rome (where Latin was the language of government and "empire", but Greek remained as a
lingua franca of the learned), so Old Anglic would have been similarly the language of government and empire, but the Old High Vilani (and Old Common Vilani) would have remained as languages of lore and culture.
After the Fall of the RoM and the descent into the Long Night (like Rome into the Dark Ages), these languages ultimately would have likely been preserved in learned circles where archives had survived and been preserved. As the Third Imperium emerged, it would be these languages (perhaps along with Sylean dialects) that would form the basis of historical records and archives, and by extension, the languages which are "looked back to" for the purposes of ancient tradition.
Since the Long Night typically did not have the opportunity to have an international network of scholars (due to the limited availability of Jump Drive), an exact analogy to Latin in Europe probably is somewhat exaggerated.
So I will concede your point that Old Anglic might make a better choice for the purposes of mottos and inscriptions.
