Polyglots are elves who can speak and understand languages instinctively and can mimic others' voices (with training), though elves with other abilities can learn to mimic as well. Polyglots often have another special ability.
All Polyglots instinctively understand and can speak in any language, including human languages. But more advanced ones can learn the art of mimicking others, as well as speaking and coercing animals.
Sophie first discovered she is a Polyglot when she was in Paris and she subconsciously spoke French without knowing the language beforehand. This occurred in the first book: Keeper of the Lost Cities.
Lady Gisela made Keefe manifest a whole new ability in Legacy. He was in a coma and woke up being able to understand ogre and turns out he was a polyglot. However, he actually manifested a whole new ability in which he can control people based on the intonation of his voice.
Lady Gisela is Keefe's mother and Lord Cassius's wife. She was the leader of the Neverseen until Fintan Pyren took over. She used her ability as a polyglot to imitate Lord Cassius to make people think he was one of the leaders of the Neverseen. This worked until Keefe knocked her hood off on Mount Everest, revealing that it was her, not Lord Cassius, who was a leader of the Neverseen. This event happened at the end of Everblaze.
She can control animals with her voice. She is arguably the most powerful polyglot the Lost Cities has ever known and has had hundreds of years of practice. She controlled the arthropleura during the attempted rescue of Prentice from Exile in Neverseen.
“The sound that came out wasn't elvin or human. Sophie wasn't entirely sure it was earthly. The clicks and chatters and flutters sounded like a dolphin crying as a million dragonflies attacked.”
Wikipedia's description of the word "polyglot" and its English language usage: https://en.wikipedia.org/wiki/Multilingualism#In_individuals