“There is not much against him, except the fact that nothing is known of his antecedents, and that he speaks too many languages for a good Englishman! (Pardon me, mon ami, but, as linguists, you are deplorable!)”
-“The Kidnapped Prime Minister”
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Based on what is explicitly stated in Christie’s own books, we can know that Poirot is fluent in at least four languages: French, English, German, and Italian. The French and English are self-evident. In Murder on the Orient Express, he conducts interviews with the passengers in three languages; this is where we learn that he is conversant in German. In the episode The Clocks, Poirot uses his knowledge of the construction of the German language to clear a certain English-speaking (but actually German) couple from suspicion.
We know he speaks Italian from this charming moment in Christie’s play, Black Coffee:
Carelli: Ah! Monsieur Poirot. Vous voulez me questionner?
Poirot: Si, Signor Dottore, si lei permette.
Carelli. Ah! Lei parla Italiano?
Poirot: Si, ma preferisco parlare in Francese.
Carelli: Alors, qu est-ce que vous voulex me demander?
Hastings: I say, what the hell is all this?
Poirot: Ah, the poor Hastings! We had better speak English.
It is also perfectly possible that Poirot was fluent in Flemish (that is, Belgian Dutch), which would seem a useful asset as head of police in the city of Brussels. But to my recollection, nothing is mentioned of this in the books. Hastings once or twice describes Poirot’s habits as “Flemish,” but the language is not commented upon. Only in the televised adaptation of The Chocolate Box is there a conversation at the Déroulard house about the use of the Flemish language.
The television series adds further glimpses of Poirot’s cosmopolitan linguistic skills in episodes such as Triangle at Rhodes, in which he ably poses some questions to the locals in Greek AND Italian…
…And in Murder in Mesopotamia, which has the following:
Hastings: “I didn’t know you spoke Arabic, Poirot.”
Poirot: “Just a few words that I have picked up, Hastings. One should never squander the opportunity that travel affords.”
In The Labours of Hercules, it is revealed that Poirot had never studied the Classics, having “got on very well without them,” at which point he is treated to a Homeric epithet in the original Greek, quoted by his friend Dr. Burton. However, in one of his labours, that of the Stymphalean Birds, he is able to solve the crime with his understanding of the average Englishman’s ignorance of foreign languages, inspiring a young man to up his linguistic game.
Poirot also admits that he knows no Russian in the case of “The Double Clue,” which is why he purchases First Steps in Russian to study the Cyrillic alphabet on a hunch. He learns enough of the alphabet to be ready when another Cyrillic clue of the same type– this time a monogrammed handkerchief instead of a cigarette case– presents itself to him in his adventure on the Orient Express some years later.
Poirot’s English vocabulary is pretty extraordinary– not terribly surprising, considering his long-term residency in London. All the same, there are several funny moments in the books in which he expresses bewilderment at some colloquial turn of phrase. For example, there’s this little conversation with Hastings from The A.B.C. Murders:
“Perhaps some convivial idiot who had had one over the eight.”
“Comment? Nine? Nine what?”
“Nothing– just an expression. I meant a fellow who was tight. No, damn it, a fellow who had had a spot too much to drink.”
“Merci, Hastings– the expression ‘tight’ I am acquainted with…”
Peter Ustinov, when playing the detective in the 1980s, took issue somewhat with the dialogue that Christie wrote for her detective. “On the printed page, Poirot is no more Belgian than Major Thompson is English. In language terms we probably see him as one of those foreign schoolmasters whose English is too correct– all very fluent and fluid and quite artificial. Remember that Poirot only puts the simplest words into French, the complex ones are always left in English.” Christie herself sometimes describes Poirot, through other characters, as appearing as a sort of “parody” of a Frenchman. For my part, Ustinov’s critique does not deter my (and presumably many fans’) enjoyment of the character’s dialogue. When Poirot uses simple French phrases like “mon ami,” it’s fairly obvious that this isn’t because he doesn’t know how to say “my friend” in English– rather, he relapses into French for comfortable phrases, or idioms that are better expressed in his native tongue and perhaps also known to his English hearers (e.g., “cherchez la femme” or “plus ça change, plus c’est la même chose”). Also, as the detective explains to Mr. Satterthwaite at the end of Three Act Tragedy:
“It is true that I can speak the exact, the idiomatic English. But, my friend, to speak the broken English is an enormous asset. It leads people to despise you. They say – a foreigner – he can’t even speak English properly… And so, you see, I put people off their guard.”