They often come up with some very strange results.
Anne Rice
In one of her books, an ancient Egyptian mummy is brought to a museum in Europe (London if I recall correctly). It turns out the mummified Pharaoh is immortal, so he unwraps and comes back to life.
When he encounters a present-day person, he speaks in Latin, and says:
"Panis!"
Coming to that point, I am immediately thrust from concentrating on the story to a bout of laughter. Not because a Pharaoh would choose Latin to communicate (the author had somehow explained that oddity), but why would he say panis? Panis, of course, is the Latin word for bread, nominative case. In other words, what the Pharaoh was saying at that dramatic moment was "Bread! Behold the bread!" But Anne Rice made it quite clear there was no bread around to behold.
The author then goes on making her own translation, and it turns out the Pharaoh was hungry and was actually asking for some bread.
Oh, you mean he meant to exclaim "Panem!", I'm thinking, finally getting it. The poor Pharaoh must have forgotten his Latin grammar during all those centuries inside the sarcophagus. He forgot that to ask for bread he needed to use the accusative case.
Or maybe it was just that Anne Rice simply looked up bread in her Latin dictionary and assumed that gave her all the information she needed.
Robert Ludlum
One of the first books I read in English after coming to America was a spy novel by Robert Ludlum.
About two thirds down, a female Czech spy encounters a male Czech spy. She mistakenly believes he wants to kill her. A very thrilling situation. Fearing for her life, she exclaims:
"Prasátko!"
Well, again, I'm completely thrust out of the story. I just keep wondering, why on earth would she cry out "Cute little piglet!"
I try to keep reading, when suddenly she says in English, "Pig!" Now that makes sense. In English, anyway.
No matter what word Robert Ludlum would have looked up under pig in his Czech dictionary, it would have come out wrong. The Czech word for pig is prase (prasátko is diminuitive). The problem is that exclaiming "Pig!" in English is an idiom. Exclaiming "Prase!" in Czech would mean, "Boy, are you dirty! Go take a shower!" Or, depending on the context, it could mean, "Quit eating so fast!"
The funny thing is, had Robert Ludlum looked up swine in his dictionary, he would have found the right Czech word for the situation.
My advice to American writers: Never assume the rest of the world thinks the same way English speakers do. And never assume your readers won't notice. If you want to use a foreign word or phrase, forget your dictionary, consult with a native speaker, or, in case of a dead language like Latin, consult with an expert.