NO SE PUEDE DECIR: "I WILL CONFIRM YOU"
How many times have you said this? A few at least? Interestingly, it is not that far from the correct version because you can actually say: "I will confirm".
Try to avoid putting the "you" at the end because that converts it into Spanglish and comes from: "te lo confirmaré" or "te confirmo".
Another alternative, which I prefer, is: "I will let you know" because it sounds a little more friendly and less formal!
Hope that helps!
Soy el director de Daniel Smith + Partners. Tengo más de 10 años de experiencia enseñando inglés a españoles.