Two words set in motion award-winning playwright Sanaz Toossi’s intricate and profound New York debut: “English Only.” This is the mantra that rules one classroom in Iran, where four adult students are preparing for the TOEFL — the Test of English as a Foreign Language. Chasing fluency through a maze of word games, listening exercises, and show-and-tell sessions, they hope that one day, English will make them whole. But it might be splitting them each in half.