Plot hole: Since the Countess was the only person to be at every dinner when the jewelry thief was in action, it takes a very special kind of idiot to not identify her as the culprit. Japp here did not need at all Poirot's acumen, but simple due diligence cross-checking the guest lists, something there is absolutely no reason he wouldn't do, and yet she is never treated as a special suspect. Also, Japp's job is described as being in jeopardy after the first 3 thefts, a 4th happens, he does not quite solve it (but retrieves at least the necklace), but he's off the hook despite the thief being unidentified, at large and with still the jewelry stolen from the first 3.
Plot hole: Much like the 1978 movie adaptation, in this version there is an absurdly short time elapsing between Linnet leaving the bridge table and the incident and murder; it's barely two minutes when she'd have to go to her cabin, do everything a proper lady of the time would do to prepare to go to bed, and fall sound asleep. That's because the witness character (Cornelia Robson here) in the novel is supposed to be droning about her boring life for a long time, but there are no gaps in her speech to imply that a long time passed - at most there's the waiter bringing a drink at the very beginning who sorta comes out of the blue.
Answer: He definitely says "Belgian", but the subtitles get it wrong and show him saying "American."
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