Is parseltongue a real language

No. It's made up for the Harry Potter books. In real life, snakes don't speak, even to each other. For one thing, their tongues are for smelling, not speaking (that's why they flick them in and out; they're sniffing). For another, snakes can't hear sounds in the air; the bones that allow us to hear speech by vibrating (they're called the malleus, incus and stapes or hammer, anvil and stirrup) are in the jaws of snakes, so they can hear footsteps by holding their heads against the ground (which is pretty easy for a snake to do).