You probably should learn a little bit. You don't have to be fluent like your character, but it's probably best that you know enough to structure grammatically correct sentences. You can't necessarily rely on Google translate or some other device to do the work for you, because it's not always accurate by all means.
You want your character to be as accurate as you can in case you come across one of those strict critics who happen to speak Latin (you never know) or maybe a reader who knows Latin. Plus it's nice to be accurate because then it makes you seem like you did your research and that your book is worth the read. I remember reading a book that was really well-written (interesting plot, funny/easy to follow dialogue), but a lot of the facts were inaccurate. The inaccuracy made the read not as great as it could've been because it was frustrating to see so many wrong things. You don't want your readers to feel that way when they read your book.
I wish you luck on your book!
