> The laws by which the states of physical systems alter are independent of the alternative, to which of two systems of coordinates, in uniform motion of parallel translation relatively to each other, these alterations of state are referred (principle of relativity)
To help you out, I think that line equates to - "Considering two coordinate systems that are moving in uniform motion of parallel translation relative to each other ("which we now conveniently call an Inertial Reference Frame"), the laws governing the movement of a physical body is independent of which of the 2 systems you choose."
Which also equates to - "The laws of physics are the same for all observers in uniform motion relative to one another "
It makes a lot more sense in German, even though I'm not that fluent in it. I searched but didn't find a better translation. It's been over a hundred years now, surely someone made the effort...