This is about Empire Of Silence (volume 1 of the Sun Eater series by Christopher Ruocchio).
In Hadrian Marlowe's formative years, his Scholiast, Tor Gibson, has taught him several languages, including the sparsely-known Cielcin language. Hadrian has some trouble with the vocabulary, but it's clear that he's pretty good at handling this complex language, which has different tones or modes of expression based on domination and other social cues.
Yet the story gives the impression that humans have never had normal interaction with the Cielcin, that they've been at war since they met (right?). Once he has finally met a group of Cielcin, Hadrian uncovers some aspects of their hierarchy and society that would have been obvious to any linguist able to interact with them long enough to document their language.
So, how was Hadrian's Scholiast able to teach him this language in the first place? Was knowledge of the Cielcin language and society acquired and documented centuries ago, in normal fashion, and then mostly erased from the books by the inquisitors of the Terran Chantry?
(I've only read Empire of Silence, but feel free to use subsequent novels if needed.)