They revitalized Broadway with ''The Producers,'' and now Nathan Lane and director-choreographer Susan Stroman have set out to save the world. In this 1974 Stephen Sondheim musical, Greek god of drama Dionysus attempts to give people hope in a chaotic time by reviving a dead poet. It's a conceit that allows for trenchant election-year satire, so too bad this is nothing more than a giant, sloppy egofest. As Dionysus, the ham-hugging Lane spends two largely self-scripted hours shtinking it up so badly that the heroic efforts of costars Roger Bart and Peter Bartlett go unnoticed. Worse, thanks to the veritable Cirque du Soleil Stroman's whipped up, most of Sondheim's belovedly complicated lyrics are lost as well. The Frogs is a Greek tragedy all right.