There’s an interesting book, published in 1930, called Masters of Mystery. An overview of detective fiction until that point, it made the interesting observation of Hercule Poirot:

Presumably for the benefit of the stupid Captain Hastings, Poirot talks in broken English—the broken English of the music-hall Frenchman… Moreover, should this music-hall Frenchman interpserse his lines with a few phrases of his own tongue, the supports of M. Hugo (the Correspondence King) expereince a superiority complex: while the embarrassed monoglots captivated by the flavour of the genuine are compelled in self-defense to join in the laugh… Poirot talks atrocious English: he cannot hold a candle to Hanaud. But it is comic: it does help the caricature. And as regards the [French phrases], Mrs. Christie has been wise enough not to expect more from her readers than a public-school smattering of the French idiom.

I’ve come to appreciate this more as I’ve been playing a mobile game, which has a built-in translator, with people from around the world. As I’ve begun picking up a few words of several different languages, I’ve come to appreciate that Poirot says in French only those words which are the first ones that anyone learns of another language if one is learning by exposure rather than in a classroom. “Hello,” “Good morning,” “thank you,” “my friend.”

It is ironic that the only things Poirot says in French are exactly the things that a French speaker would begin to say in English after two days of living in England. But there is an excellent reason for this irony: they are also exactly the things that the average Englishman has a hope of knowing in French, so that the lack of translation is not a problem for the reader.

You can contrast this with Dorothy L. Sayers who will have Lord Peter and Harriet Vane say entire sentences in French without translation, and most of us need to simply guess at the meaning, shrug our shoulders, and move on. (She’ll do the same with Latin, too.) The way that Poirot uses a few recognizable French words and a bit of French grammar or literally-translated idioms may be unrealistic, but it does a much better job, I think, of getting across how foreign he is while keeping him intelligible.

(And, of course, it’s also interesting how he explained it himself at the end of Three Act Tragedy.)