I think people are forgetting that Poirot WAS expected to see through this.
Yes but Poirot was not supposed to become suspicious until McQueen told him that Ratchett didn't speak any foreign languages -
after Ratchett had already been found dead.
The OP actually does raise an interesting point. None of the conspirators knew that Ratchett had spoken to Poirot in the dining car, and that their conversation might have clued Poirot in prematurely to the fact of Ratchett not being a French speaker. So they actually took quite a big risk by having McQueen yell out in French in Ratchett's compartment. Poirot could have realized right away that the voice he heard likely did not belong to Ratchett, and since he also knew Ratchett had received death threats he might have gotten up to investigate right then and there. Which would have been awkward, to say the least.
However I don't think Ratchett's mispronunciation of Poirot's name by itself would necessarily indicate to Poirot that Ratchett didn't know any French - it would more likely just suggest that he was bad at French pronunciation. And since McQueen's French pronunciation isn't very good when he yells out about the nightmare, Poirot probably doesn't give it a second thought when he hears it, which enables the murder to proceed as planned.
ETA: Interestingly I just realized that a clever clue about this is planted earlier on in the film, when McQueen is told by Pierre that he will have to share his room with Poirot, and he complains "Vous avez faites un erreur", showing us before the murder occurs that he does speak French.
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