Six years after its establishment on a professional basis, the Washington Ballet is going international with performances in at least four countries, the company announced yesterday. Trips starting early this summer and next fall will take the troupe of 17 dancers (plus two apprentice dancers) to major performance sites in Spoleto, Italy, and Paris, with visits before and after to Colombia, Luxembourg and elsewhere. These are the company's first appearances outside the United States since its reorganization in 1976 along professional lines.

"I honestly feel that this is the finest group of dancers we've ever had," said founder/artistic director Mary Day yesterday. "The recognition has been coming along for Choo San, for Amanda, for Bonnie, and the company as a whole, and now that it has all come together, it seems almost natural that producers would want to show us to audiences in Spoleto and Paris and wherever there are ballet-curious people."

Day's references were to the troupe's resident choreographer and assistant artistic director Choo San Goh, 34, who, since coming to the Washington Ballet in 1976, has amassed an international reputation, and whose ballets are now to be found in the repertories of American Ballet Theatre, the Joffrey Ballet, the Alvin Ailey American Dance Theatre and many other major troupes, as well as the Washington Ballet; to Amanda McKerrow, 18, who last year became the first American ever to take a gold medal at the prestigious international ballet competition in Moscow, with her partner, Simon Dow, also of the Washington Ballet, winning an award for partnering; and to Bonnie Moore, 17, who took first prize in her category at the Prix de Lausanne competition last January.

The company plans to leave June 23 for Barranquilla, Colombia's second largest city, where the troupe has been invited to inaugurate a new theater, the Teatro Municipal, with performances June 25 and 26. It would then leave for Spoleto's Festival of Two Worlds, founded by Gian Carlo Menotti and celebrating its 25th anniversary this year. The Washington Ballet will be the season's opening attraction,. performing July 1-4, and all three of the company's recent prize winners--McKerrow, Dow and Moore--will be on hand, along with Day and Goh. The primary program for Spoleto will include Goh's "Fives" and his newest ballet, "In the Glow of the Night."

The company will reassemble in Washington in mid-August to rehearse for fall touring, beginning with performances Sept. 16-19 in the troupe's "second home," the University of Virginia in Charlottesville. Between Sept. 27 and Oct. 3, the company will perform a full week at the Champs Elysees theater in Paris, with guest artists Peter Martins and Heather Watts of the New York City Ballet sharing the program. The company has further performances scheduled for Oct. 5 and 6 in Luxembourg; others elsewhere in Europe, possibly totaling up to five weeks, are still in negotiation. The '82-'83 season in Washington will begin with performances Nov. 10-14 at Lisner Auditorium.

McKerrow will perform at Spoleto, but as yet her plans for the fall are not fixed (the company, though it would certainly be delighted to have her in the lineup, is not guaranteeing her presence in Paris or elsewhere). Speaking for her on Friday (she was in Jackson, Miss., dancing at a benefit for the international ballet competition to be held there this summer), her father, Alan McKerrow, who acts as her manager, said: "Amanda is considering various possibilities for the fall, but at the present time isn't committing herself to anything specific--she's looking forward eagerly to Spoleto." In the meantime she will perform her first full-length "Sleeping Beauty," with Dow as partner, at the Pendleton Festival in Illinois in June.

Goh and Dow have other travel in store, too. Dow will depart after Spoleto for the Australian Ballet, where he'll dance leading roles as a guest artist through the end of the year. Goh will be in Paris late this month staging his "Leitmotiv" for the Paris Opera Ballet; from there he'll proceed to Israel to mount a work of his for the Bat-Dor troupe.

The company has undergone extraordinary growth in recent years. Over the current season alone, the number of repertory performances, excluding the annual run of "Nutcracker," has risen from 16 (in 1980-81) to 50. Since the fall of 1980, the company had its debut in New York (and a return engagement there last month); was awarded a three-year $100,000 challenge grant from the National Endowment for the Arts; appeared at the prestigious Jacob's Pillow Festival in Lee, Mass.; and greatly expanded its domestic touring with the help of a contract with Columbia Artists.