When foreign words are included in English text, they are italicized.
He had a certain je ne sais quoi aspect that I found utterly fascinating.
While all the ones in your second paragraph are so common that I wouldn't even italicize them, "explicite" is not a commonly used foreign expression in everyday English. If you can simply substitute in the English word "explicitly" I would do that, unless it's important to show the person was using a French word.

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