They are able to understand all languages, but prefer to specifically speak in Celestial.
It comes across similar to Sylvan, in that those of the untrained ear only hear choir music an various types & cadences of bells. Unbeknownst to most, the Celestial language works primarily off the listener. The instruments and melodies portray how the listener truly responds to the subject at hand. It is far more difficult to lie in celestial or lie to a creature speaking in it, imposing disadvantage on deception checks. If the listener responds to the creature speaking in Celestial in lies, half-truths, or misdirection, tones of the celestial speaker will become discordant & shift - growing dissonant and unnerving.
The speaker of Celestial's own cadence won't even change, but the reactions of the person they're talking to is an easy indicator of whether or not the listener has any sins, regrets, or denials about the topic at hand.