How can Hadrian Marlowe speak Cielcin when humans haven't had social contact with the Cielcin?

Question

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.)

Answer 1

I thought this was a plot hole as well. But you could explain how they know about some of the language by the huge amounts of cameras recording everything in this galaxy. The Cielcin communicate to each other on the battlefield with words which the cameras would record. Linguists would certainly begin to figure out some of the language from these interactions.

Answer 2

In one of the later books (can't remember which one exactly) it's revealed that Lord Cressian Powers captured a Cielcin Worldship intact after the Second Battle of Cressgard, even in Marlowe's time this find accounts for 70% of Cielcin artifacts in the Empires possession.

This,combined with the fact captured Cielcin tend to be relatively docile (for reasons I won't spoil) would explain how the Empire was able to gain a solid grasp of the mechanisms of the language (if not the context), at least solid enough for a Scholast like Tor Gibson to pick up and pass on to his ward.