In the Harry Potter books, people who take Polyjuice Potion get the voice as well as the physical appearance of the person they're imitating. Without needing to make an effort, they sound exactly like the person whose face they bear (source: Harry getting Goyle's voice in CoS).
What effect does this have on their accent? Or, on a larger scale, their language?
For instance, if someone Polyjuiced themselves into Hagrid (OK, you can't Polyjuice into a half-giant, so let's say a human with the same accent, to keep the pedants happy), would they immediately start speaking with a strong West Country accent? If Viktor Krum Polyjuiced himself into an Englishman, would his English suddenly be perfect? Going even further, if someone who only spoke English Polyjuiced themselves into Krum, would they then be fluent in Bulgarian? If someone Polyjuiced into Voldemort, would they be able to speak Parseltongue?
And if the answer is yes to any of these questions, could the language effects last beyond those of the potion itself? E.g. if I Polyjuiced myself into Krum enough times, would I eventually be able to speak Bulgarian just from memory?
(I'm excluding film canon for this one, because of course in the films people who take Polyjuice keep their own voice unless they make an effort to disguise it.)