This is weird!
If you're a long term fan of the franchise, you'll know that Pikachu has always been verbally limited to "Pikachu" and "pika," occasionally to a "pika pika."
According to @tigerwang, Pikachu isn't actually speaking words during the scene. "He can't really talk!! It's when they emotional moments they have a connection and can mentally understand each other," he tweeted.