According to Omniglot.com, Navajo (Diné Bizaad) (Navaj, an Athabaskan language spoken in Arizona and New Mexico in the USA) first appeared in writing in 1849 in the form of a Navajo word list published in the Journal of a Military Reconnaissance by Lt. James H. Simpson.
At the beginning of the 20th century missionaries began producing religious texts, dictionaries and grammars in Navajo. But each missionary invented his own spelling system; therefore many different ways to write Navajo emerged.
But in the 1930s, John Collier, the head of Indian Affairs and Willard Beatty, the head of Indian Education, decided that a standard Navajo alphabet was needed and they commissioned John Harrington, Robert Young, William Morgan, and Oliver LaFarge to create such an alphabet and to publish Navajo language materials.
By 1939 the new alphabet was finished and during the 1940s the first bilingual primers, Navajo language children's books, a modern dictionary, and a monthly newsletter were published. However, it was regarded by the tribal members because of their anger at Collier's policies on livestock reduction.
What is "goodbye" in Navajo? It's hágoónee’. It is commonly used in parting, and it roughly means “okay/alright then” in English. It’s almost like saying, “Okay, things are settled. See you later.”
Want to learn more? Check out some useful phrases in Navaj at Omniglot.com.