The Asia Society is offering a free DOCUMENTARY SERIES about important political figures of the past generation each Thursday at 12:15 in the theater of the Embassy of Australia, 1601 Massachusetts Avenue NW. Reservations are suggested for priority seating and may be obtained by calling 387- 6500. The subjects to come: Indira Gandhi, Kukrit Pramoj, Douglas MacArthur, Mahatma Gandhi, Mao Tse-Tung and Ho Chi Minh. On Tuesday, the National Gallery will begin a free series called SPANISH CINEMA, booked to supplement the El Greco exhibition, with a 6 p.m. showing of Luis Bunuel's "Tristana." It will be repeated at the same time Wednesday through Saturday. The features scheduled for subsequent weeks are Carlos Saura's "Cousin Angelica," Victor Erice's "The Spirit of the Beehive," Cesar Ardavin's "Lazarillo" and Rafael Gil's "Don Quixote," a 1948 production that co-starred Rafael Rivelles, Juan Calvo and Fernando Rey. All films are in Spanish with English subtitles. The new comedy NIGHT SHIFT, previewed two weekends ago, gets another advance exposure this Sunday night at the AMC Skyline, K-B Congressional Plaza, K-B Langley, Showcase Fair City Mall, Showcase Bradlick, Showcase Mercado and Tenley Circle. It opens theatrically next Friday, joining a new Chuck Norris adventure, "Forced Vengeance," and the drolly titled import FRUITS OF PASSION, a film version of Pauline Reage's "Return to the Chateau," the sequel to her sadomasochistic whopper, "The Story of O." Booked exclusively at the K-B Janus, "Fruits" represents a potentially mind- boggling collaboration between the Japanese director Oshima, best known for an enervating sex classic of his own, "In the Realm of the Senses," and the German actor Klaus Kinski, cast as the chief libertine. The Inner Circle is hosting an open-ended test engagement of a new documentary feature, IN OUR WATER, dealing with environmental pollution. New opening dates for a couple of summer releases previously postponed: "The Sword and the Sorcerer" on August 6 and "The Road Warrior" on August 20. The Friends of Fred Schepisi's BARBAROSA, which turned into a small but gratifying success in Washington two months ago after getting the coldest of openings from Universal, may be pleased to know that it's finally getting a New York booking -- a stop-gap exclusive at Cinema I starting this Sunday, due to the instant collapse of Claude Lelouch's "Bolero." Now back in Los Angeles, Schepisi is reported to have several potential projects in various stages of negotiation or pure speculation. The most surprising and yet tantalizing: a belated movie version of the musical "Seesaw," with Liza Minnelli as Gitel Mosca and Tommy Tune perhaps doubling as choreographer and supporting player, presumably recreating his role of the gay hoofer. "Baby, It's You," a Paramount acquisition I had confused with another comedy on the studio's release schedule, turns out to be a more promising title than I'd imagined. Scheduled for release this October, it's the second feature written and directed by John Sayles, who made such a gratifying debut with "Return of the Secaucus Seven" a couple of years ago. The subject of his new film, made independently and recently picked up by product-hungry Paramount, is a high- school romance set in the middle Sixties and enacted by Rosanna Arquette and Vincent Spano.