I am looking for an analogue to Latin for the world of D&D. To modern English speakers, Latin is not well known, but since it is still taught in some areas and it supplies a lot of vocabulary to English, English speakers can have some luck puzzling Latin out. On top of that, many other modern languages are much more closely related to Latin than English, so if someone knows e.g. Spanish or French, they may have an advantage here.
Is there any language that might serve a similar role for the languages spoken in D&D? I am playing 5e in a homebrew setting, and I would like to create some language puzzles that doesn’t just involve having a player who happens to know the language used.
Basically, if Common is English, what language would be Latin? What languages would be closer to that language, like Spanish or French?