Japan
Germany and Japan
The question is unanswerable as asked since it does not define who the "enemy" is.
Japan
Japan & Germany .
Their language was indecipherable to the enemy
yes
I would say Germany. Although Japan was a big enemy.
During World War 2, California was part of the United States, so any enemy of the U.S. would, by extension, have been an enemy of California. For example, Italy, Japan, or Germany.
The USSR
yes
The code that was never broken by the enemy was based on the language of Navajo. Neither the Japanese, nor the German could break the code words. The code was used by people that fought in the Pacific war during world war 2.