I learned Latin over a span of four years. I was somewhat fluent, but was only really interested in the cursory understanding - since it is a useful language to help understand other romantic languages - and its use in binomial nomenclature was of interest to me.
It's been over 5 years since the last day I was learning Latin. I've so rarely used it that I am no longer remotely fluent in it - so I probably can't help you.
That being said, how is perfectly conjugated and structured Latin possibly important in your story?
Does your editor think that readers are going to go beyond the internet Latin to English translators to verify its authenticity and accuracy?
Heck, I've read some poorly structured Latin that amounted to someone doing this...
Taking the sentence, "I really like ice cream and pie and birdies"
and translating it into Latin that, if re-translated back to English by a fluent speaker would probably read more like, "The ice cream birdies and pie I really like."
Despite that, It was hardly a big deal. You do realized that 95% of people that use or say any Latin phrases don't even fully know what they are saying. They are really just parroting a common Latin phrase that they have a basic understanding of. That or they are reading it off a page. That or it is a species' name in the binomial classification system.
I am genuinely curious why the editor would request this UNLESS Latin is an incredibly integral and widespread feature in your book, rather than a device used occasionally when a character is calling out some spell they use.
Edit:
Good luck finding someone fluent in Latin (When I say fluent, I mean can hold a normal conversation in Latin with another person with no glaring errors). There are not many, and their help may not come particularly cheap.
Unless I am misunderstanding the extent of the Latin in your book.
Edit 2: But maybe I am wrong
