There is no such diagnosis listed among the
Personality Disorders in the DSM:
http://en.wikipedia.org/wiki/Personality_disorder
Dissociative Disorders are listed and described briefly here:
http://en.wikipedia.org/wiki/Dissociative_disorder
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Charlie Chaplin was able to convincingly fake an operatic singing voice in one of his films. He said "I just
acted like i could sing opera." or words to that effect. A person in a kind of trance might be able to mimic a well trained singer, yes.
A person in a trance might also be able to mimic, verbatim, sentences in a foreign language they didn't understand, as long as they've heard these sentences spoken at some point in their past.
The ability to actually generate original, coherent sentences , appropriate to the situation, in a language you've never learned, though, has got to be fiction.
I saw Val Kilmer interviewed after he played Jim Morrison. He told a story of how he would try to practice his Jim Morrison on some of the remaining members of
The Doors who were on the set as technical advisors. One of them told him to stop it because he was so much like Morrison it creeped him out.
So, a person can, indeed, do an uncanny imitation of another person, but they would have to have been exposed to that person in person, or have seen film of them, etc.