How we cite our quotes: (Chapter.Paragraph)
Quote #4
Civil engineer Philip Johnston […] convinced Marine brass that the Navajo language—unwritten and spoken by only those who had lived among us Navajos—could be the basis for an unbreakable code. (9.19)
Here we get an understanding for why the Navajo language was chosen as the basis of the secret military code. It's unwritten and spoken by few people, so chances that the Japanese will manage to break it are slim.
Quote #5
How could we, twenty-eight of whom had never worked with the military, develop a code robust enough to be used in battle? (10.10)
This quotation expresses the self-doubt that the new Navajo Marines feel about developing the code. Can they live up to the difficult task that they've been assigned, especially given that most of them have never even had any experience in the military?
Quote #6
On that first day, we decided to use an English word […] to represent each letter of the English alphabet. Those words would then be translated into Navajo, and the Navajo word would represent the English letter. (10.13)
It's a good thing that Chester and his buddies have a good grasp of both English and Navajo, as they need both to develop the secret military code. While Chester was torn between the two languages growing up, here we see the two languages coming together.