The movie star's translator, and in on the whole deal, La Valle is good at putting on airs and sounding important as he and Devereaux do their best to get their hands back on the drug car.
La Valle seems relentlessly cool and French and cosmopolitan, showing Charnier around the auto-yards prior to the police auction, and at ease while translating for Devereaux at the pier-side press conference. But once they get the car back and it's transferred to Charnier, Devereaux says sayonara and we don't see him again.
Lost in Translation
In the original script, La Valle seems to be more of a "tool of information" type, telling Charnier all about Ward's Island, and showing him where cars are purchased for Charnier's company and processed before being shipped over to Marseille. But this type of explanation ended up on the cutting room floor—The French Connection, like Popeye Doyle, is more about action than info.