Is there an original manuscript available? The first published edition is available online.
Depending on what you want to find, the people at the University of Sussex's Newton Project may be able to help you.
What language was it written in?
Same language as the title.... Latin.
Cambridge has Newton's own copy of the first printed edition, with extra blank pages for handwritten annotations and corrections for the second edition, e.g.:
Aha, he tweaked the Third Law for the second edition:
Separate names with a comma.