An interesting Stack Overflow question popped up about auto-translating one document into another when edits are made. Most Google services have things called "triggers" which run functions after some kind of event - opening the doc, making a change, etc.
A Google Doc does not have access to the
onEdit trigger, so there is no way to automatically run the translation unless you put it on a timer trigger, and that's a waste of resources, especially if you're not constantly updating the document. But, you can link two documents together via the ID and push changes made using a custom menu.