Voice coaching

actor design the voice and speech of a character in the context of a cinematic, stage, radio or animation voiceover production.
The voice coach’s job is to do a research on languages, dialects and their speech patterns, to prepare training materials based on the way languages sound, to instruct and work on lines with the actor.
A voice coach will give the actor feedback focusing on issues of credibility, consistency, and clarity.
In a word, voice coaches teach actors how to imitate a language that an actor doesn’t speak.

American accent because the movie and TV market in the United States is larger than either of those countries, and the demand for British and Australian accents in the US is lower than is the demand for American accents.

was born in West London, England. He gained his popularity as a comedian, writer and an actor. But his all roles are different.
For example:
Borat
The Dictator