It is Kyuubi who teaches him to speak.
In the beginning, Naruto is alone. As his limbs lengthen and his skull strengthens, as the fat melts away from bone and the red in his eyes melts into pure blue, he stays alone in his windowless room. Now and then, people appear - tall, cold people in bland clothing and blank features. They leave food and water and sometimes touch him to change his coverings. They do not speak except for a soft murmur. For the first few years of his life Naruto knows himself only as demonchild, even if the word means nothing to him.
If it weren't for Kyuubi, Naruto would be mute. He wouldn't know how to speak, how to communicate and his only expression of self would be violence. He wouldn't speak to people, wouldn't make them better. Without his words, Naruto would be nothing.
But Naruto had Kyuubi. While his body sat alone and silent in his windowless room, in his mind an infant sat by the towering bars of a cell. And every time he appeared a thick, luxurious tail (so soft, and today the only thing that truly calms Naruto is plush toys and the fur of animals) would unfurl through the bars and wrap around him.
In a voice, tender and smooth, like a blade wrapped in satin, Kyuubi would talk. At its most lucid it detailed death. The way you could see the moment life fled the body. The spray of blood from some areas of the body and the languid trickle from others, like a clean mountain spring. It detailed its life - the swath of destruction it had cut across the landscape. At its worst it described the sights of its last moments of freedom; the blond human falling, dead before he hit the ground. The human had bounced and Kyuubi still savoured the sound of bone cracking and the scream of grief from the summon. When it details the blond human its voice grows hard and it upsets the infant Naruto. Before long he is flinching at the mere mention of his father and the irony keeps Kyuubi entertained for months.
At its best, Kyuubi talks about freedom. It talks about a wind stirring its fur, about the roar and crackle of fire, of the way reflection of flame dances in the eyes of the few survivors. It talks about the sheer joy to be found in running - the push of paws against solid ground, the stretch and pull of muscles and the reach for more speed, more.
Naruto feels most at peace at himself when he runs, even more so when he does it on all fours. He refuses to dig too deeply into that, too afraid of what he might unearth.
Before long the infant began to mimic the sounds, clumsily trying to twist his mouth into the shapes Kyuubi's muzzle makes when it talks. It's impossible to do and so Naruto improvises until the sounds coming out are the same as the ones it hears.
When Naruto is reading the scroll he stole so many years later, he has to improvise. He slots words he knows in place of those he doesn't and comes up with something that works for him that allows him to replicate himself. As it watches hundreds of its host's body fill the clearing, Kyuubi remembers the attempts at mimicry that Naruto has forgotten. The way he had spoken of death and destruction with a grin, knowing nothing more than he'd succeeded in his task.
Kyuubi remembers. Perhaps it will tell its host someday, it reflects and the images that spring to mind make it settle with a rumble of contentment deep in its chest.
Kyuubi's deepest disappointment (barring the seal that put it where it is and prevents it from freedom) is when the old man comes and introduces Naruto to the outside world - to happiness and joy and a love and respect for fellow humans.
It'd almost gotten attached to its host. Perhaps the man had done it a favour.