In the last Off-Broadway show I saw here in NYC (I Was Most Alive With You), they had sign-language interpreters signing for their respective actors on an elevated platform. It was a cool effect that suited the subject matter (it was a play about deafness whose main character is deaf), but obviously not practical for every play.

In another (Light Shining in Buckinghamshire) there were subtitles projected at the front of the room. You could get used to it, but it was somewhat distracting and not ideal. This approach seems much better!

Both plays were incredible, btw. Light Shining in Buckinghamshire blew my mind and I highly recommend picking up a copy if you haven't read/seen it.