Technically, I don't believe that they have int at all. So they're not, strictly speaking, literate. However, I would probably rule that, as an extension of the casters' will, they can write things down for him. (Taking dictation would be a reasonable use of the spell).
As precedent for this, consider non-intelligent undead such as skeletons. They have no intelligence however they are able to respond to commands such as "destroy the door on the left" or "kill the man with the wand" or "grapple the warrior on the left with the one-handed axe." All of these commands require a reasonable degree of comprehension (most animals, for instance would not reliably be able to follow any of those commands). The skeleton's lack of intelligence refers to a lack of independent will and tactical ability rather than a lack of perceptual skills or ability to understand orders (indeed, traditionally, skeletons are able to follow reasonably complex orders like "Patrol these halls at a speed of 20 feet counterclockwise until you have circled twice then reverse course and patrol clockwise once then start over; kill anyone who enters this building unless they bear the symbol of the Doombringer, or speak the password [Insert Password Here]; once you have killed them, dump their bodies in a pile in front of the ghoul pit and resume your patrol." Similarly, an unseen servant can be told to dust a room (and knows how to dust), separate every strand of straw in a haystack until it finds a needle (and knows the difference between hay and a needle), etc. It's not unreasonable to assume that it can be told to write and will know how to write (as long as the caster knows).
They couldn't, however, follow written instructions, translate, or do anything that requires independent thought.