Because the navajo languages are only spoken but never written.
To keep language on and on you need people to speak it, and people to remember it. However, from my memory, the number of navajo tribes arent much and the language is pretty hard to learn since it is only a spokent language, so there are no written records for it.
You should sprinkle it throughout. Otherwise, the reader will be uninterested if all of the interesting details are in one part of the story and stop reading it.
