Just to clarify for people; The jibberish talk has already been confirmed. He clearly states they will "keep what people already love" and lists "gobbedy goop speaking" during this talk:https://youtu.be/8ulJeGQ0Krk?t=8m47s
With that in mind, here is how it is most likely going to go down:
You will play the game. Every character talks in nonsense. When you reach the credits you will see "Jon Jafari" listed as a "noise maker" much like in the credits for Banjo-Kazooie.
Now, would that really bother you that much? Would it take you out of the game? Did you ever notice who was playing who in the old Banjo-Kazooie games until they spoke out about it after the games release? It's not like any character has ever been portrayed with the actor's normal speaking voice. You didn't know Grant was playing Mumbo until he started talking about it in interviews about ten years after the game was released. You're most likely not going to notice Jon.