31: The Son of man is, of course, the Lord Jesus Christ. The coming in glory with the angels refers to his second coming. This will occur at the end of the age, which the disciples had asked about. He will be coming to establish his kingdom on earth physically for one thousand years. The throne he occupies will be in Jerusalem. From there he will rule this world.
32: The nations here refered to are the gentile nations. Individual gentiles will then be judged for their fitness to enter the kingdom. This is not the final judgement of the living and dead.