In Hollywood and at New York University’s film school, people had told David Riker that no one would want to see a movie shot in Spanish without a single professional actor. But his film of real immigrants dramatizing their stories in their native language — the sweatshop on the silver screen — sold out show after show, prompting the Quad to extend La Ciudad’s run from one week to three months. So many immigrants arrived with their entire families that the theater waived its policy of refusing admittance to children under 10.