I don't know latin but am fluent in French and Italian. When authors incorrectly use foreign words it bothers me. Another thing that bothers me is the the author using the clichéd meanings of foreign words. Not that they would know every nuance a foreign word has but that doesn't sound like your case.
I'm a strong believer that if you use a foreign language, you'd better have someone who's a native speaker check and make sure you're using those words properly. To this day, I still can't bring myself to pick up Shadow and Bone because of how Bardugo uses Russian. As a Russian speaker, I went "nope, nope, nope!"
Everyone has basically chimed in, but definitely use the correct endings! For people like us, who know and can read Latin, it would be something that would personally give me enough to discredit the reliability of the book. If you want a reference to how Latin is well used in a novel, check out The Raven Boys by Maggie Stiefvater. She incorporates a ton of (correctly used) Latin conversations and phrases. I double checked them myself ;)
KelseyCross, it's not that I don't know how to use Latin (I've spent years learning Latin and classical Greek for my job), it was a matter of making sure that people wouldn't get testy or icky. I use a lot of languages in my book (Russian, traditional Chinese, and Latin off the top of my head), and while it's easier to get away w/ messing with some languages (really, how many ways can you transliterate a Chinese character -- off the top of my head there are only 3-4 systems...), Latin has declensions so the mixed reaction from CPs and betas worried me. Thankfully it's settled now, hooray. =)