The Nyong’o siblings’ casting sounds like a gimmick, but the payoff is exquisite: the twins, dazed at their reunion, establishing the truth of their identity as they talk to each other in Swahili. For the audience, too, this “Twelfth Night” is a kind of reunion — with the Delacorte, a vital space that New York theatergoers depend on, and missed. It speaks our language.