Parseltongue is a fictional language spoken by snakes in the Harry Potter series of novels (author: J. K. Rowling), whose known human speakers include the Lord Voldemort and Harry Potter. This language has, however, not been publicly described by Rowling and has no known form.
Indeed, it is unlikely that a dictionary or a grammar of Parseltongue was ever written even in the Harry Potter wizarding world itself. The reason for this is that Parseltongue is not as much a learnable language as an innate magical ability. Either you are born a Parselmouth, in which case you can speak and understand Parseltongue without ever learning it, or you are not, in which case you will never ever be able to speak or understand Parseltongue.
What a Parselmouth hears when he listens to a snake (or another Parselmouth) are the natural hissing sounds of the snake (or, respectively, their imitation by the Parselmouth), and at the same moment, he magically knows what the snake (or the Parselmouth) wants to tell him. When he speaks Parseltongue, the Parselmouth subconsciously 'translates' his thoughts into snake-sounds. The knowledge of the grammar and the lexicon of Parseltongue is subconscious and cannot be told to someone else or written down.