My book has a few words from ancient languages I've used. Mainly to add authenticity, we follow the Celtic characters in modern English (apart from the occasional un-translatable concepts), but other races such as the Saxons or people speaking Latin are kept in Old English and Latin. Do you think this is a bad idea? A good example of what I mean: “Spare this mongrel, and let him run off and tell the other wéalas of this kingdom [...]" Saxon word meaning “inferior race” or “foreigner”. Later became bastardised into Welsh, giving the people of Wales their name. Taken from my novel. The POV character speaks both Celtic and Saxon so can understand the rest of what he's saying.