There are more French expressions in Death on the Nile than in any other of the Poirot stories. Here is a list of all them, with some indication of why Poirot uses them:

À merveille!: ‘Excellent’;  usually used by Poirot as an adverb (i.e. ‘we proceed à merveille’)
À votre santé: ‘Your good health’ (Poirot toasts the Otterbournes, one of whom is a dipsomaniac)
Ah, non!: ‘No way!’ Reaction to Dr Bessner after the discovery of Linnet’s body
Ah, vraiment! ‘Indeed!’ Poirot is suspicious of Pennington’s motives
article de luxe: ‘Luxury article’ – in reference to Jackie’s pistol

Bien: ‘Ok, good, fine’ (variously)
bon Dieu!: ‘Good God’
Bonne nuit: ‘Goodnight’

Cache: ‘Hiding-place’ (for Mrs Otterbourne’s booze)
Ce cher Woolworth: ‘Dear old Woolworths’ – Poirot considers a cheap handkerchief
C’est de l’enfantillage!: ‘This is sheer childishness!’ Musing on the letter J scrawled in blood on the cabin wall.
C’est vrai: ‘That is true’. To Race

Cette pauvre petite Rosalie: ‘That poor young girl.’ He also refers to ‘cette pauvre Madame Doyle’ but it is Rosalie Otterbourne that he feels sorry for.

Écoutez, madame: ‘Now listen, madame…’ The beginning of a long speech to Linnet.
Eh bien… : ‘Well….’

empressement: ‘Eagerness, alacrity.’  Poirot observes the maître d’ finding him the best table at the beginning of the book
en verité: ‘In truth’

femme de chambre: ‘chamber-maid’

jeune fille: ‘young girl’

la politesse: ‘politeness, courtesy’
le roi est mort – vive le roi!: ‘The king is dead, long live the king’: referring to Linnet. Joanne Southwood had previously (and more accurately) referred to her as “la Reine Linette”

les chiffons d’aujourd’hui: ‘today’s chiffons’: the French expression ‘causer chiffons’ used to mean to gossip about clothes; Rosalie and Jackie have been comparing lipsticks, which Poirot sees as its modern equivalent.

Ma foi!:  ‘Indeed!’ This is followed by “madame, that was close” when the boulder narrowly misses Linnet. The nearest literal translation to this would be the old-fashioned “i’ faith”

Mais oui, Madame: ‘Indeed it is, madame’ (to Mrs Allerton, when she proclaims the lovely night)
Mais c’est tout: ‘But that’s all’
Moi, qui vous parle: ‘I, I am telling you’ – Poirot being emphatic to Race.
mon ami: ‘my friend’ (to Race). An endearment often addressed to Hastings in other books
Mon cher Colonel: ‘My dear Colonel’ (Race again)
Mon Dieu!: ‘My God!’ When Simon complains that Jackie isn’t being reasonable
Mon enfant: ‘my child’ – addressed to Jackie when he tries to give her advice

Nom d’un nom d’un nom!: ‘In the name of God and all the saints in heaven!’ Poirot sensibility is outraged by the J scrawled in blood on the cabin wall.

On ne prends pas les mouches avec le vinaigre:  ‘You don’t catch flies with vinegar’. Poirot finds something suspicious in the nail-varnish bottle. An English version of this is “you catch more flies with honey than vinegar”.

Parbleu!: ‘Heavens above’ – followed by “but I am not the diving seal!” To Mrs Allerton
Précisément: ‘Precisely, exactly’. Used by Poirot a great deal in all his stories
Peut-être: ‘Perhaps’. To Colonel Race

Quel pays sauvage: ‘What a wild country…’ to Race
Qu’est-ce qu’il y a?: ‘What is it?’ In response to an exclamation from Race.

Sacré: ‘Damn!’ Unusually strong language for Poirot. The French translator Marie-Josée Lacube changes it to “Enfin!” in her version of Poirot Investigates.

Tenez!: ‘Now look here!’ (to Rosalie Otterbourne, who has been fiercely criticising Linnet)
Tiens, c’est drôle, ça: ‘Hello, that’s a bit funny’… when Simon unwittingly lets a clue slip
Très bien, Madame: ‘Very well’ (to Mrs van Schuyler)

Une qui aime et un qui se laisse aimer: ‘One who loves and one who lets himself be loved’

Zut!: ‘Blast!’ Poirot fails to find the necklace

And finally, Poirot quotes to Jackie a Belgian poem written in 1905:

La vie est vaine

La vie est vaine              Life is a play
Un peu d’amour            We love a bit
Un peu d’haine              We hate a bit
Et puis bonjour             And then it’s good-day
La vie est brêve              Life is so slight
Un peu d’espoir             We hope a bit
Un peu de rêve              We dream a bit
Et puis bonsoir              And then it’s good-night