From English, at the Roundabout’s Todd Haimes Theatre. Photo: Joan Marcus “Sometimes I think you can only speak one language,” says a character in Sanaz Toossi’s English . “You can know two, but…” She trails off, her brows knitted and her eyes far away, leaving something unexpressed in words but plenty articulate in the reverberant air. It’s an especially poignant moment in a play that’s full of them while still maintaining its essential lightness. English doesn’t sink toward soppiness; it —