Why does Orwell discuss tranlating the work of such writers as Shakespeare, Milton, and Swift into Newspeak?
I assume that you are talking about the part in Chapter 5 where Winston is talking to Syme about Newspeak and things like that. If so, I think that the major reason that Orwell discusses this is to show how much the Party is in the business of changing the past.
When works like those of the writers you mention are translated into Newspeak, they have to be changed completely. There would be no way to actually translate them because the ideas in those works are totally against the Party's values. As Syme says
The whole literature of the past will have been destroyed. Chaucer, Shakespeare, Milton, Byron -- they'll exist only in Newspeak versions, not merely changed into something different, but actually changed into something contradictory of what they used to be.
By doing this, the Party will be changing the past. They will be putting new and different ideas into the "mouths" of those past writers. This shows how far they are going to try to change their society and their past so that it all matches up with Party doctrine.