The Navajo tried to copyright the word Navajo in the mid 70's but the court  did not let them because the word was too common.
The only way to get into legal trouble is via the Indian Arts and Crafts Act which says you cannon claim something is 'Indian made', which Navajocoin is not. no one has or is claiming that this coin was made by the Navajo Nation.

There is no legal problem here.