My rough translation :
Then Peter turned and saw John, who was the dude that was all up in Jesus' face going all like, "Bruh who's going to betray you?".
Peter, seeing John, said to Jesus, "Bruh what's the betrayer gonna du?"
Jesus replies with, "If I want him to remain until I come, what is it to you? Follow me."
And then I get confused with the double negative, so bear with me for a little.
Then went this saying abroad among the brethren, that that disciple should not die: yet Jesus said not unto him, He shall not die; but, If I will that he tarry till I come, what is that to thee?
^So Jesus is all walking around, saying the betrayer ain't gonna die (is he re-affirming his previous statement), but if the betraying dude remains until Jesus returns and says, "...what does one make of that?"
I'm not sure Jesus says that's a prophesy as more of a suggestion, if anything. O.o