Plotholes in DBZ

When the saiyans (nappa + vegeta)  arrive on earth why can communicate with the earthlings? Why dont they speak some alien saiyan language?

Why does vegetas hair color change from brown to black by the time he travels to earth?

The reason why vegeta's hair was brown and then black was because Toei didn't know color vegeta's color since the manga wasn't in color so they guess it. and Bardock had pyshic powers and its possible for him to go back in time. but the Episode of Bardock is a spin-off and no one is sure if its canon or not