I'm a librarian who loves anime, manga, and reading a wide variety of genres.
This book has prompted me to google Chinese nobility and titles. It struck me as odd that the kingdom of En (which does have some Western aspects, but I think is mostly supposed to be similar to ancient China) would have dukes and viscounts. The Wikipedia pages I've seen (such as the non-imperial nobility section here) indicate that this is not necessarily a funky translation, though. It still feels weird to me.
After I'm done with all the books, I'm going to rewatch the anime, just to hear what words people use and see what the subtitles are like. For example, I vaguely recall Book 2's "lamia" being "nyokai" in the anime subs. "Lamia" has connotations that didn't seem to fit what Sansi was, although maybe "nyokai" has similar connotations for Japanese speakers? Guh. Reading stuff in translation can be frustrating sometimes.