Foreign Soiled

My boyfriend was reading The Brief Wondrous Life of Oscar Wao last month, and he said it would have been nearly impenetrable if he didn't know Spanish. On the other hand, what are the odds that all those award committee jury members spoke Spanish?

How much foreign language is too much?

My most recent story is set in a French-speaking country, so there are a few French words here and there (ie, "mon petit" is said at one point). I either made sure the words were obvious in their context (such as the above endearment, spoken from an adult to a child she's babysitting) or I provided a translation right away--and I never used more than one foreign word in a scene.

The more sophisticated ways I got around using foreign languages in a foreign country while maintaining authenticity were:

1. My narrator was American, so sometimes she couldn't understand what was being said, either.

2. When people spoke English to her, they were usually translating from French--so I figured out how to say their line in French *first* and then wrote the direct English translation of it. For example, I would use French-English cognates, even if a native English speaker would be more likely to use the cognate's synonym. Sometimes I would also change the order of words, if another order would be *much* more natural in French. I tried to never use American idioms or word order that would be *supremely* unnatural in French.

3. Once, when the narrator is out of commission and two French-speakers have to talk to one another, they exchange one French phrase. I don't translate the phrase, but it's one character asking another to do something--and the second character immediately does what the first asks him to do. At that point, the narrator is in a really hazy, bad place, so I figured a little confusion would be fine. The context definitely required that the characters speak in French, but luckily they didn't have to speak much nor about anything abstract.

Of course, little to none of this would have been possible if I didn't have a relatively strong background in French; I can't imagine doing it in any other language. How much background knowledge IS necessary, though? Was *all* my knowledge of French coming into play, or does one only need to have a feel for the language?