That depends. My Welsh teacher was a linguistics student, and he said you could get by for most everyday purposes in a foreign language with the vocabulary and grammar rules you'd pick up within the first weeks. It's not fluency, but it will do, and a writer could probably get by after that with only the occasional, "what's the word for..." or passing remark that goes misunderstood to remind the reader. (though couching the rest of the dialogue in minimal vocabulary would help. I'd be thrown again if serious nuances cropped up too soon.)

Of course, I can think of exactly one protagonist of mine who isn't at least bilingual for just this reason.