In most languages words are as convenient labels for things and concepts. In the true Celestial language the word is the same as the thing itself, giving one with knowledge of the word control over it. Indeed there is so much information contained within your “average” celestial word that glimpsing the writing or hearing even a whisper can burn out the senses of most creatures.
With Celestial requiring so much power to use, even gods themselves would use something of a dialect in their day to day lives. Both Draconic and Infernal are said to have been heavily influenced in their form and structure, perhaps explaining why those two languages are more easily able to coax life from Khandrian Starstones and are so heavily used by spellcasters.