Known as the Tongue of the Divines, Celestial is practically a dead language, with only those studying the Pantheon really devoting a lot of time to it. It is spoken (arguably quite poorly) at religious services, and many religious tomes are written in Celestial. Those growing up in religious households tend to dread their lessons in the language.
Some argue that 'Celestial' is a bit of a misnomer; there is New Celestial, the language as it is known today, and Old Celestial, the true language of the Gods, from the times where actual divine beings existed in Iralas. To pretend that deities speak the 'pidgin Celestial' known to mortal races today is, in the eyes of linguists, a joke.