Atlantis: The Lost Empire/WMG

The Heart of Atlantis is an alien AI.
To be precise, the Heart is a intelligent power source. Similar to the WMG below, the Heart is an empathic power generator, and is capable of intelligent thought. If some speculation is correct, and the Heart is actually alien tech, then in the crashlanding on Earth its systems would have been heavily damaged, capable of only utilizing ancient emotions and thoughts as a power source. That is why it needs a host, preferably one of royal descent. Being royals in such a society, they would know things a commoner wouldn't, therefore the Heart uses them like an updating software, allowing the Heart to assess the situation and direct its power towards where it is needed.

The Heart of Atlantis is the Disney Multiverse counterpart to the Blue Lantern Power Battery
Remember Kida's words of "All will be well"? The sentience within the crystal is Adara, the Hope Entity. The power allows for healing and longevity within the Atlantean citizens, though at the occasional requirement for a royal family member to Ascend to A Higher Plane of Existence and temporarily house Adara for a massive job before becoming part of the blue energy.

The Heart of Alantis is the Gaia Eywa.
It is a "life force." It gives help and protection to the Atlantians. When they die, they ascend to it (Kida gets her lost bracelet from there.) However, the man mentioned at the start of the film "You've doomed us all," tried to attack it,(they killed their mother and the city sunk because of it. She did have enough kindess to let them live, but let the culture die. The rest of the world turned away from Gaia, and she was forgotten with Atlantis and replaced by other things people worshipped.

The Heart of Atlantis only chooses women of the royal family as its vessels/sacrifices.
And only the males of the royal family are carved into stone. They are the protectors of the feminine power that the Heart of Atlantis is comprised of.

Milo is Daniel Jackson
The movie begins in the distant past. Flash forward to the 20th century. Milo/Daniel, a young, poor, orphaned linguist, has uncovered evidence of the existence of a great civilization in the past, which was previously thought to be just a myth. Despite his success in deciphering the ancient language, his ideas are rejected by peers. However, a mysterious woman steps in and offers him a chance to prove his theory. Once Milo/Daniel meets the benefactor that the woman is working for, and explains his ideas, he is invited to join an expedition to where no man has gone before -- a rag-tag team led by a straight, no-nonsense military man. Once they arrive to the destination from which they can possibly never return, they discover an underdeveloped, dying society that has abandoned and forgotten their written language -- and Milo/Daniel becomes a translator between the two cultures. After the team realizes that getting back home will be much more difficult than they initially thought, Milo/Daniel is unjustly blamed for getting them stuck there. However, He is aided by the local leader's beautiful daughter, with whom he falls in love. She leads him to the ancient inscriptions that reveal the history of their civilization. Milo/Daniel's team rallies the locals to fight in a heated battle, during which the protagonists find a way to destroy the villain's ship without also wiping the city in the process. After the villain is defeated, the expedition returns home -- but Milo chooses to stay behind, to learn the ways of the natives and to live Happily Ever After with his newfound love.

IT'S THE SAME THING!

Written language in Atlantis was near-exclusively restricted to scribes or royalty
And those scribes were blinded or killed when Atlantis sunk. How else do you explain the ability to read entirely disappearing within a generation?

The crystal took away the Atlantian's ability to read
The king says to Milo that "In my arrogance I sought to use it [the crystal] as a weapon." The crystal was incredibly pissed off by that, and so it decided to take away the Atlantian's warmongering abilities. One of the things it did to accompish this was to take away the Atlantian's ability ot read so that they would no longer be able to operate their war-machines, but instead would have had to rebuild their civilization's traditions and things from the ground up. If you have a machine that catches fish, but you don't know how to use it anymore, and you can't read the instrucion manual, then you're going to have to spend more time learning how to catch fish a different way so you don't starve is my thinking. By the time Milo got to Atlantis, the people had shifted their priorities, and the crystal let them relearn how to read because they were no longer a war-race.

Only the royal family was made to live until they were Really Seven Hundred Years Old after the Maebelmac.
The civilians continued to age at a relatively normal rate for people who have no infection or permanent injury and a slight boost to their "don't fall apart after the quality of your copied cells starts to degrade" ability, and the royal family (having a direct connection to the crystal instead of merely living under its influence) got a version of the last ability that was imperfect but turned Up to Eleven. Learning of this is part of why Milo isn't worried about an immortal old coot running around the surface and being Crazy-Awesome-emphasis-on-the-"Crazy" for too long, Who Wants to Live Forever? and the easy removability of the crystals notwithstanding.

All the Atlanteans turn white because of their age.
The processes that turn hair white take the same amount of time with Atlanteans as they do with us. If all of them age at the rate Kida does, than a year is something like 400 or 500 years. Considering a lot of people go white at 50 or 60, all the Atlanteans could easily go white before their first "year".

The Heart of Atlantis uses the crystals to absorb and store memories the people of Atlantis don't currently need.
That's why the thousands of years of memories don't drive them crazy, and also why they aren't a population of super-geniuses.

The Heart of Atlantis was the source of the Atlanteans' splendid linguistic ability.
You can't speak languages simply because they descended from yours. Native speakers of Latin would not be able to speak French, for example. But as the Atlantean empire grew and absorbed other nations, the Heart of Atlantis "learned" their languages, which allowed the Atlanteans to speak them. When it was locked away, the Atlanteans lost their ability to read because it was tied up with the Crystal, but retained their gift of tongues.

The Atlantaeans forgetting the their language and how to operate their machinery is a side effect from their longevity.
This article states that one of the reasons immortality would suck is that your ability to remember would degenerate the longer you live due to the sheer amount of information you would possess. Considering the Atlantaeans are over eight thousand years old and the crystals likely prevent mental decay they subconsciously forget most of their memories with a few exceptions (remembering their names and what happened on the day Atlantis fell, for example). Skills they needed on a daily basis like hunting and crafting were retained through repetition, essentially relearning the skills over and over again while writing and machine operation fell to the wayside as unnecessary. This is why the Atlantaeans become Experts when flying their craft in the final battle as they experienced mild deja vu while piloting their fish-craft. Their daily routine is probably so similar until the expedition arrives it all mashes into a single set of memories (wake up, hunt, make baskets and clothes, etc.)

There were other civilizations around the world as advanced and powerful as Atlantis.
The reason the king attempted to use the crystal as a weapon of war. This was a time when highly advanced empires like Atlantis ruled the Earth, and were bitter enemies. The explosion that destroyed Atlantis would only set off a series of events that would eventually wipe out the rest of the empires.

The Atlanteans forgetting how to read their own language is a side effect of them all being so old.
Think about it. The human brain can only remember so much. If you were over eight thousand years old and in a stagnant culture, if you didn't practice it, you would lose to ability to read.

The death of the crew was planned by the Commander.
Because there's no way hundreds of people would turn a blind eye to his evil shenanigans and he wouldn't share with all of them. So he planned for all the non-essential to die.

The Heart of Atlantis was given to the Atlanteans by the Precursor Race from Treasure Planet.
The race that built Treasure Planet is possibly one of the most advanced cultures in the Disney Animated Canon, so they could have made the Heart of Atlantis and either lost it on Earth, or they could have given it to the Atlanteans for some reason.