CREATING A CELESTIAL CREATURE
“Celestial” is an inherited template that can be added to any corporeal animal, aberration, animal, dragon, fey, giant, humanoid, magical beast, monstrous humanoid, plant, or vermin of good or neutral alignment (referred to hereafter as the base creature).
A celestial creature uses all the base creature’s statistics and abilities except as noted here. Do not recalculate the creature’s Hit Dice, base attack bonus, saves, or skill points if its type changes.
Celestial creatures dwell on the upper planes, the realms of good, although they resemble beings found on the Material Plane. They are more regal and more beautiful than their earthly counterparts.
Celestial creatures often come in metallic colors (usually silver, gold, or platinum). They can be mistaken for half-celestials, more powerful creatures that are created when a celestial mates with a non-celestial creature.
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Magical Beast Type: Magical beasts are similar to animals but can have Intelligence scores higher than 2. Magical beasts usually have supernatural or extraordinary abilities, but sometimes are merely bizarre in appearance or habits.
Features: A magical beast has the following features.
—10-sided Hit Dice.
—Base attack bonus equal to total Hit Dice (as fighter).
—Good Fortitude and Reflex saves.
—Skill points equal to (2 + Int modifier, minimum 1) per Hit Die, with quadruple skill points for the first Hit Die.
Traits: A magical beast possesses the following traits (unless otherwise noted in a creature’s entry).
—Darkvision out to 60 feet and low-light vision.
—Proficient with its natural weapons only.
—Proficient with no armor.
—Magical beasts eat, sleep, and breathe.
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These are taken directly from the srd. From this, celestial creatures are simply creatures from the upper planes, who have abilities because they are outsiders. Magical creatures are natives of the prime material plane which have been altered by magic to be something 'more'
I'd rule that Celestial creatures understand Celestial as their base language. To them, Celestial is their common. If they have the intelligence for it, they may take additional languages. Same thing for creatures from the infernal and abyssal planes respectively.
Magical beasts, if they have the intelligence for it would understand common as their base language. Or perhaps sylvan would be a better choice.