While reading Burmese Days, something surprised me. I had to read that book with a dictionary close at hand. Now, I'm not saying I have the best vocabulary there is, because that would be patently false, but I know that when it comes to the English language, I've got some game. I am not accustomed, as it were, to needing a glossary to get through a text.
However, the words that were tripping me up weren't English. The book is littered with Urdu, some words that the Internet implied were Nepalese, some thing I'm 98% sure were Burman, and more than a little of what the text called "Hindustani." I feel confident enough in myself to admit that my "Hindustani" is not good and my Burman is even worse. (If you speak either of those languages, I feel I ought to shiko to you, or maybe just a handshake, since I'm a tad rusty on the bowing, too.)
In the context of the book, the use of so much non-English makes a lot of sense. After all, the book takes place in Burma (you know, in case the title didn't give that away), and the characters who are speaking have spent many years in Burma, most of them, if they're not natives and/or not English speakers. Moreover, sometimes, there's just no proper English equivalent for some things that happen outside the English speaking world. The Burmese fits.
Still, I know that I would not feel comfortable doing something similar in any of my books. No matter what language my characters are speaking, or where they are standing, I would worry a great deal that I was tripping up the reader by splashing around all sorts of words I knew they probably wouldn't know the meaning of. (Bonus points if you can define the words dacoit, lakh, and longyi without a dictionary. I couldn't.)
If I were going to use a word I didn't think my reader would know, my instinct would be to define it somehow in the text. Otherwise, I feel like I'm throwing my reader out into the sea without so much as a life-preserver. 'Oh, here you go, dearies. Hope you can keep up. Oh, you don't speak Portuguese? How sad for you.' The definition is a life-raft for the reader so that they can stay hooked into the story without having to pause and wonder, 'wait, what is a chokra and why do they keep talking about them.'
One author I thought did this very well was Lemony Snickett. He had a habit of defining words and expressions in his books in highly amusing ways. For example, "his[Olaf's] face fluttering as he tried to decide whether to come clean, a phrase which here means 'admit that he's really Count Olaf and up to no good,' or perpetuate his deception, a phrase which here means 'lie, lie, lie.'" I this because it's funny, and because it allows the author to use the phrasing he'd like without sacrificing reader comprehension. Heck, it even adds a teeny bit of educational value, but don't tell the kids that. ;D
How do you feel about the use of non-standard English or non-English or obscure/complicated English in books? Do you use it? Do you define terms you think might be unfamiliar to your reader?