Christina Milian Frenching 'Voice' Coach Matt Pokora on a Yacht

Christina Milian's done some work with "The Voice" ... but nothing as hot as what she's doing now with one of the show's judges ... in France. she's shoving her tongue down on a yacht off St. Tropez.

The dude she's tongue kissing is Matt Pokora -- France's answer to Adam Levine, even down to the tats and frosted 'do. He's not only a huge French pop singer ... he's held a spinning seat on two 'Voice' spin-offs in the land of baguettes and fromage.

Not sure how they met, but they've been chilling in the South of France for a few weeks now.

Looks like Christina's mastering the tongue language.