Regarding English, I think that the first treatise was Abraham De Moivre's treatise The Doctrine of Chances (1718).
For the discrete case, see page 7 :
"If the Events in question are $n$ in number and are such as have the same number $a$ of Chances by whcih they may Happen..."
And see also William Emerson (1701 – 1782)'s The Laws of Chance (1776), page 3 :
"AXIOM I. In computing the number of Chances, it is supposed that all Chances are equal, or made with equal facility."
And page 4 :
"SCHOLIUM. As $1$ represents a certainty, or when an event has an infinitely great probability of happening, so $\frac 1 2$ represents an equal probability [emphasis added] for happening or failing. For $1 − \frac 1 2 = \frac 1 2$."
For a good overview of the early history of probability in general and of equipossibility in particular, see Ian Hacking, The Emergence of Probability : A Philosophical Study, Cambridge UP (2nd ed 2006), Ch.14, page 122-on.