In the books (Animorphs) they can speak telepathically when they’re morphed into animals, so the brackets are to show that form of communication. It’s often used in situations they can’t risk speaking aloud in, or to plan privately between each other.

One of the perks of their shapeshifting power is short-range telepathy, which they use to communicate on missions while in animal form. In the book, telepathic dialogue is marked in <Angle brackets, like this> to differentiate it from "Verbal dialogue, which comes up only slightly more often."

It's French