While in college I took French to satisfy the foreign language requirement for my degree. One of my instructors relayed a story of his first trip to France back in the early 1970’s. A group he visited with was curious what it was like back in America, and at one point they asked him about, what sounded like, “lazy peas.”
He contemplated this for a few seconds and questioned his fluency in the language. Laissez pees? That still didn’t make sense. And then it dawned on him. They were asking about les hippies, known in English as the hippies.
Using a foreign language in writing can add authenticity to a story if a character is from another culture. But the writer will need to make some decisions. Should you stick to well-known terms like oui (pronounced wee for you non-French readers)? Dare you throw in something most readers won’t understand?
Like any good crime, that will depend on your motivation. If you just want to add color to a character or the setting, use familiar words or brief phrases. Or have another character respond in the native language of the story, thereby divulging that lazy peas spoken of aren’t meant for human consumption.
Maybe you do want to impart some mystery (although readers of that language won’t be stymied). But since a mystery is meant to be solved, you’ll need to make the meaning apparent at some point.
You can also establish a set-up for using terms in another language. For example, in Wail of the Tempest, Alexia is a Cajun and bilingual. She makes a comment about somebody getting the fremeers, and translates to the reader it means “getting the willies.” Later in the story she has an unpleasant interaction with someone that gives her the fremeers. The reader has already been acquainted with that term and understands what it means.
As in all good writing, if it doesn’t add to the story, don’t use it. Otherwise the readers will just be confused by lazy peas running around, and they’re baffling to many people as it is.