mot juste vs diction

mot juste and diction both are nouns.

Word	Noun	Adjective	Verb	Adverb
mot juste	Yes	No	No	No
diction	Yes	No	No	No

As nouns, diction is a hypernym of mot juste; that is, diction is a word with a broader meaning than mot juste:

Other hypernyms of mot juste include choice of words, phraseology, phrasing, verbiage, wording.

mot juste (noun)	diction (noun)
the appropriate word or expression	the manner in which something is expressed in words
	the articulation of speech regarded from the point of view of its intelligibility to the audience