It only takes a few years at most to learn a new language, and if you were present for the entire development of those languages, you would be fluent. Odds are that the first two immortals are fluent in nearly all languages, or at least on their continent, it really depends on if they moved. as for accent, I've developed an accent that isn't my natural one before from talking to people with that accent and that only took half a year or so, so odds are that the immortals accents are that of the people around them.