WHEN the Kennedy Center tribute to Germany marches into town next spring, the National Gallery of Art and the Library of Congress will join in as part of the big brass band, while several other Washington arts organizations will be beating the drums behind it.In an unprecedented cultural parade, there will be a month's worth of attractions (April 5 - May 9) for the crowds. They range from the sublime, Bach's St. Matthew Passion, to frisky German-dance classes in the polka, schottisches, and waltz - all at the Kennedy Center. Politics and culture are braided together in this festival, as they often are in Washington. James Wolfenson, chairman of the Kennedy Center for Performing Arts, says that for this international festival "we should take as our very first subject the unification, the linkage of East and West Germany, ... to pay tribute to a heritage which has had such a significant influence on this country." He says it was backed by "the spiritual and financial" resources of the Federal Republic of Germany. Jurgen Ruhfus , the German ambassador, notes "We are grateful to the American people, whose history and traditions are inextricably linked to the idea of freedom, for having supported German unification wholeheartedly." The National Gallery salutes German artist Kathe Kollwitz (May 3 - Aug. 16) with an exhibition of 100 prints, drawings, and sculptures. Through its music division, the Library of Congress will display manuscripts of German masters: Bach, Beethoven, and Brahms, as well as a Mozart serenade, Schumann's first symphony, and part of the Wagner's opera, "The Flying Dutchman." Goethe-Institut Washington (German Cultural Center) is also sponsoring a number of events, as are nearly a dozen arts groups. Among festival backers are the International Monetary Fund and the US Information Agency, which Mr. Wolfenson describes as being "a partner in this with us." Also at the Kennedy Center, the legendary Stuttgart Ballet will dance a premiere of Tchaikovsky's "The Sleeping Beauty," in a new production staged by prima ballerina Marcia Haydee, and Tchaikovsky's "Eugene Onegin" with choreography by John Cranko. The tribute will also take over the Eisenhower Theater for the Berlin "Theater des Westen," a cabaret-review reviving the German nightclub style. The National Symphony Orchestra will salute German composers like Wagner, Schumann, Haydn, Weber, Brahms, Mozart, and Handel.