Nacre translates to "mother-of-pearl," which is the iridescent inner layer of certain shells, often used in jewelry.
In the song, Benjamin Biolay sings, "Quand tu portes Ă l'envers ton collier de nacre" (When you wear your mother-of-pearl necklace inside out). It's a beautiful and somewhat uncommon word that evokes a sense of delicate beauty and perhaps a touch of eccentricity, fitting the unique and enchanting description of the person in the lyrics.
"Les joues roses" is Benjamin Biolay's close-up portrait of someone he's completely taken with. He moves through her in pieces: her voice, her skin, her hands, the small habits and little vices that make her who she is, and keeps landing on the same detail, her rosy cheeks, as the thing he loves most.
The song stays intimate and a little playful rather than grand. He calls himself her whore, her circle, her gang, a devotion that leans on slang instead of poetry. Later he widens the lens: she's unique, magical, a masterpiece, and even the way she wears her mother-of-pearl necklace backwards moves him. It's a love song built from noticing, not from big declarations.