In summer, the superlatives roll off the tongues of theater producers like a stream of fighter planes leaving an aircraft carrier.

The subjects of these superlatives are, of course, next season's plays - 18 of which have been announced, coincidentally, by Arena State, the Folger Theatre Group and Ford's Theater.

"It's possibly the most beautiful play I've read in 10 years," says the Folger's Louis W. Scheeder about Arnold Wesker's "Love Letters on Blue Paper," which arrives at the Folger in February.

On the other hand, Robert E. Ingham's "Custer," which the Folger Group will present at the Kennedy Center's Terrace Theater in October, is merely "a marvelous play" with "lots of wonderful music [cavalry songs and civil war songs]" that promises to be "beautiful to look at."

"I always use superlatives," says Scheeder.

Arena Stage, for its part, wants it known that 12 American theaters were in the bidding for David Hare's "Plenty," and Arena won out. Produced at England's National Theater the season before last, "Plenty" concerns an Englishwoman's obsession with the one moment of high drama in her life - her brief career in the French resistance during World War II.

"Plenty" will open in April under the director of Arena's David Chambers.

Another highlight of the Arena season will be the Isaac Bashevis Singer Eve Friedman dramatization of Singer's "Teibele and Her Demon," substantially as produced at Minnesota's Guthrie Theater last year. Announced and then withdrawn this spring because of rights dispute, "Teibele" is now set for Arena's Kreeger Theater in October.

Songwriter Jule Styne, incidentally, plans to present "Teibele" on Broadway later next year.

And Romanian director Liviu Ciulei will be back with "Emigres," by the Polish playwright Flawomir Mrozek, at the Kreeger in April.

Arena's eight-play subscription season is now finished, but several non-subscription plays may be announced later. The Folger has set six of its seven productions, with a new play still to be chosen for a spring appearance at the Terrace (in keeping with the Folger's recently announced agreement with the Kennedy Center).

Ford's Theater has only half a season ready, but probably will lead off with the Washington premiere of Michael Cristofer's "The Shadow Box" in October, according to producer Frankie Hewitt. This Pulitzer Prize-winning play, by a Catholic University alumnus, is about three cancer patients. Hewitt hopes to have Richard Chamberlain direct, and to use members of the cast of Chamberlain's Boston production of the play.

Will Rogers' 100th birthday will be observed with the fourth appearnace of James Whitmore in "Will Rogers USA" at Ford's in November. Next comes a still-to-be-chosen version of "A Christmas Carol" for December, and Hermione Gingold in "Side by Side by Sondheim" early in 1980.