I just won an award for a story written in three languages: Nimipuutimt, Proto-Celtic, and English. So, I strongly advocate for using the languages necessary to tell the story. Obviously, there are people out there who enjoy foreign words in their books because there are a good number of novels which contain foreign languages, some of them quite famous. Some are footnoted, some translate in text, a few don't do either, leaving it up to the reader to figure out what was said. Heck, there are famous novels who use entirely fictitious languages and still manage to be successful. To say something won't get published simply because it includes foreign languages displays a good measure of faulty logic. There are a lots of books written entirely in English that never get published, so I would wager that language is not the limiting factor. You should write the story you would want to read and if that includes foreign languages so be it! Besides, this may be your only chance to be in the 1%, so you'd better take it!
I have to question your understanding of "faulty logic" in this context... You're saying that if something without X doesn't get published, then clearly X isn't what keeps things from getting published, which... is silly. Like, let's say I wrote a novel-length work that consisted entirely of the lines "I like Coca-Cola" over and over again. And someone said it was unlikely to get published because of its constant Coca-Cola statement. Then you would say there are lots of books that never mention Coca-Cola at all that never get published, so clearly writing a book that consists only of "I like Coca-Cola" isn't a problem. It's not good logic. I mean, I have no idea whether using different languages in a book will be a barrier to publication or not. But accusing others of faulty logic when you're making a completely illogical argument yourself isn't a good idea.
I'd use these words sparingly and with good explanation and context. Fantasy writers make up words all the time (such as to explain classes of people, places, beliefs, concepts, events in history, etc). They just have to handle it well so that a reader can keep track of what's going on. The same can apply to words that already exist but won't be known to the reader, I think.
