PARIS, Monday, April 22 — "Start the party!" shouted Jean-Marie Le Pen as the last words of his victory speech, and a roar went up, drowning out the fusillade of Champagne corks at the buffet. Then he reneged, and asked that all first sing the national anthem, "The Marseillaise": "Let's go, children of our country, the day of glory has arrived. . ." The crowd packed into his overheated headquarters in the Paris suburb of St. Cloud poured it on in full throat on Sunday night, straight through to the words about engaging the foreign enemy and "irrigating our fields...