Abyssal, Celestial, Infernal, Axiomatic

Though often spoken of separately, the languages of the major extraplanar powers are actually four "modes" of an overarching tongue that encompanses the precepts of all of them. Often referred to as the Language of the Spheres, the difference between the modes is more one of tone and expression than of vocabulary. Different writing systems are used for each of the four alignment modes as well, though they sound similar enough when spoken that anyone who understands one mode can probably get the gist of any of the others, and may be able to speak haltingly in it.
Examples of this language are extremely rare, and are usually only found in very obscure books or the inscriptions of old temples and ruins.
Speakers of one of the four alignment modes who have proficiency in Arcana can communicate in the other three modes imperfectly and may be able to decipher the writings of the other three with moderate difficulty.