Latin and Greek are classical, 'dead' languages.
In the time of J.S.M. they were languages used by academics and upper classes regularly enough that in his circles he and many of his peers had early exposure.
Hence that scene in Monty Python's Life of Brian.
Still fun today: https://www.youtube.com/watch?v=Mip30YF1iuo
That there are no native speakers doesn't mean there are no tutors that speak it.
In the time of J.S.M. they were languages used by academics and upper classes regularly enough that in his circles he and many of his peers had early exposure.
Hence that scene in Monty Python's Life of Brian.
Still fun today: https://www.youtube.com/watch?v=Mip30YF1iuo