The performance is part of a triple-bill of 20th-century operas that provides one of the centerpieces of this year's contemporary music festival at Tanglewood. Sharing the program, which will be repeated tomorrow afternoon, are Paul Hindemith's Hin und zur‹ck and Stravinsky's Mavra. According to the Tanglewood Festival/Boston Symphony web site (www.bso.org), the operas, full staged, will be sung in English.