Question about Arrival (2016) (Warning: Heavy Spoilers!)

Anyone who fully understands the language, has the ability. Or so I understood it, anyway.

It's all a bit vague, imo. But indeed Amy was the first to truly understand it, and has then spread the ability.

It's unclear why humanity has to learn it, other than that the squids are going to need humanity's help later on. Not that much of a problem for me, I love the movie.

(Sorry I cracked open your "spoiler", but the *entire thread* is clearly marked as "spoiler" material...)

I think you're wrong.

The film is explicitly endorsing an old (and discredited) view in linguistics called the Sapir Whorf hypothesis, which posits that which language we speak radically affects the way in which we see the world - and crucially, the way in which we are able to see the world. While the characters are beavering away at understanding the aliens' language they explicitly talk about the Sapir-Whorf hypothesis, and what it might entail.

Amy Adams learns to see the world the way the aliens do (from a god's-eye, atemporal perspective) because she has fully learned their language. Anyone else who takes the trouble to do this, and it is a lot of trouble, might do the same thing.

Now maybe, even at the time of the event with the Chinese general we see at the end, nobody else in the world has as yet learned the aliens' language; but that's an irrelevant detail: Amy Adams's character is fully capable of teaching it to someone prepared to make the effort, and no doubt in time there will be more people picking it up.

she was captured & replicated by the pod-people, & returned to earth to spread the coming invasion of the body snatchers...