While it might seem difficult for an author to write a book in the voice of a kid with language difficulties, Stork totally nails it. What better way to show us Marcelo's struggle than to let him speak in his own strange and beautiful words?
Stork gives us great insight into Marcelo's perception of the world when he says, "Even when [Jasmine] is angry, like at Juliet for example, you can tell that the anger does not affect her. The reason I can tell is that her breathing never alters. A person who is truly angry has physical reactions that last for a while, even after the event that caused the anger is gone" (8.27). Marcelo has to use logic to figure out feelings, which is completely alien to most of us. If someone gives us the silent treatment or the side eye, most of us can tell they're mad or annoyed. Marcelo can't, though, and when we see his struggles through his eyes, we're way more drawn in than if Stork just told us about him in the third person.
It's as if suddenly, our brains have to work like Marcelo's. And that's quite the change for most of us. But without that change, we wouldn't be able to experience Marcelo in the Real World they way we're supposed to—as Marcelo.