noted, Latin phrases are occasionally quoted in Dominic Flandry's period, not only by Flandry himself but even by two extraterrestrials. This entails that some knowledge of Classical literature has survived into the Terran Empire. What I forgot to mention, however, was that the Latin language had been put to more practical use a few centuries earlier. Merchants of the Polesotechnic League had used "League Latin" for interstellar communication.
Why Latin with all its complexities and irregularities? Surely Interlingua, mentioned in James Blish's Cities In Flight, or Esperanto would have made more sense? Or is "League Latin" Interlingua? Esperanto has simple regular grammar, Roman letters and phonetic spelling with Latin and other European roots, e.g., Greek kaj instead of Latin et. "Father" and "mother" are not patro and matro but patro and patrino and "Lady" is Lordino - so the grammatical regularity is complete.
The Terran Empire mirrors the Roman Empire even down to having slaves but here is one logical difference: harems are guarded not by eunuchs but by non-humans! However, sex is possible between human beings and some other intelligent species, as we learn in the Young Flandry trilogy. Indeed, when aliens are described as both humanoid and bisexual, then we have to ask whether this might not be possible - although interstellar hybrids as in Star Trek and some Superman stories make no sense.