Good question - I had a look at David Frith's book
The Slow Men to investigate. He makes the point that for an underarm bowler, leg-spin is an easy and natural action, so the first leg-spinner is forever lost in time. But he names a man called Lamborn, who played for Hambledon around 1780, as the first off-break bowler.
As for the first international spinner, it rather depends on what you class as the first international. Spin bowling was well developed by the 1850s, when bowling was at its brief roundarm phase, and America v Canada was first staged then, followed quickly by the first English tour to Australia in 1861-62. If you take the 1877 Melbourne Test as your starting point, I believe several slow bowlers took part in that match. At that time it was common for one to open the bowling.
Edit: if you take
this match as the first international, most of the bowlers are not categorised, but Crossley, who had most success, is classed as an off-spinner.