Voice Coaching

coach who helps an 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.

US and Canadian entertainment businesses. However, some dialect coaches name

themselves dialogue coaches or dialogue directors. A term voice coach in the US is a synonym for singing (or vocal) coach, a person who trains singers.
In the UK voice coaches are generally called accent coaches, because voice coaches in the UK are people who work with breathing, voice and text, but not with the language phonetics.

Hugh Grant's nutty roommate in Notting Hill, once described vocal

work as "taking your mouth to the gym.“ This is something that Bob Corff, one of the most demanded voice coaches, stresses to all the actors he works with before they even start working.

"This is not the most fun you could ever have. This is 'I’m going to master something in a very short amount of time, and I’ve got to really focus on putting my mouth, my lips, my tongue, the back of my throat in positions [they've] never been in.' Everything in your body and ego is saying this isn’t right and you have to learn how to do it enough times so you get that that is what it takes to make that new sound."

first step is to go through the vowels - A,

E, I, O, U.
Then actors have to go through melody exercises, just like how singers have to go through scales. If you pay attention, you'll notice that every language has its own cadence, a sing-songy quality which gives native speakers a particular rhythm. Just think of Italian — can you say "Mamma mia!" without singing it like the spaghetti chefs from Lady and the Tramp?
After that, actors go through the script and practice as many times as needed to get the right accent. It’s very important to keep the accent through every rehearsal and to pay attention to the articulation every time you speak.

(This Dialect Coach Can Transport You With Her Perfect Accents)
https://www.youtube.com/watch?v=ljf1s1rzJ0A

(American Theatre Wing. Dialect Coach)
https://www.youtube.com/watch?v=fn5EspfaCqk (Dialect Coach Guesses Who Is Faking An American Accent)