Hebrew is a familiar medium for Walter Turnbull’s vocalists. “We were singing in Hebrew 10 years ago,” says the founder and director of the Boys Choir of Harlem. Psalms are a constant part of the group’s repertoire. “We’ve always sung in Hebrew.”But the world-traveling choir had even more opportunities to practice the language in recent months. In May, Turnbull and 48 of his singers made their first trip to Israel for 10 days of performances, workshops and tours.