Quote from : Ender's Game Book

They must talk to each other directly, Ender, mind to mind. What one thinks, another can also think; what one remembers, another can also re-member. Why would they ever develop language? Why would they ever learn to read and write? How would they know what reading and writing were if they saw them? Or signals? Or numbers? Or anything that we use to communicate? This isn’t just a matter of translating from one language to another. They don’t have a language at all. We used every means we could think of to communicate with them, but they don’t even have the machinery to know we’re signaling. And maybe they’ve been trying to think to us, and they can’t understand why we don’t respond.