Draconic

Draconic is the language of the dragons, however it is often also used by draconic related races (The Dragonborn and Kobold) along with lizardlike species, such as the Lizardfolk, however dialects vary greatly between said species, and communication of all but the most basic of ideas and emotions is nearly impossible. 
Speech in Draconic almost always consists of hard consonants and sibilants that sound like hissing when spoken, with particular emphasis on the first syllable of a word. 
Few can read or write in Draconic, and the alphabet itself was created well after the languages inception, as Dragons had little to no use for a writing system. Thus text in Draconic is rare and those who can read and write it even more rare, making it an excellent choice for keeping the meaning of a text secret.