It does not say "judging JUST the twelve tribes". It also does not say that everyone else (the nations of the world) will not be judged or by whom they will be judged.
Let me ask you this: how will the Gentile Nations be blessed during the Millenial reign of Christ? Or do you believe there will be actual Nations on this earth for Christ’s 1000 year Kindgom? And just what does the covenant God made with Abraham mean? One more question: Why twelve disciples? Why not 11 or 15? Or when Judas betrayed Christ and went to his own, why was he replaced by Matthias? Why was it necessary to replace him when they did?