Rufus Wainwright wants the opera, “Prima Donna,” to be in French; its would-be commissioners — the Met and Lincoln Center Theater — insisted on English. Wainwright said another stumbling block was the date of a potential production at the opera house. The earliest the Met could offer, he said, was in 2014. “They work on that sort of scale; I wanted to get it out as soon as possible,” he said, adding wryly, “because I’m an impatient pop star.”