I've seen people mention Python, Node.js, Go, Ruby on Rails, etc. Note, we want someone experienced in leading a team as well as experienced in the language which that team writes. At the end of the day the back-end lead, presumably highly qualified & willing for such a position, would have the final say here, but we'll have this poll to see what people can do. If you're OK with multiple languages, just reply to the thread with a list and vote for your favorite.