“His return to the stage on Friday — to headline the season opener at Tanglewood, the Boston Symphony Orchestra’s summer home in the Berkshires — is being closely watched not only by his fans, but also by the music industry. … The select group of artists who can still sell out concerts on the strength of their names includes Yo-Yo Ma, Itzhak Perlman, Joshua Bell and Renée Fleming — and Mr. Lang.” Michael Cooper explains what kept Lang Lang away from playing and what’s at stake now.