Seems like they would have been just as useful. I know some other nationalities were used in a sample scale, but nothing approaching the pacific campaign.
Well the Navajo were used exclusively in the Pacific, primarily because the Navajo code talking program was a Marine program. And the marines did not serve (with individual exceptions) in Europe.
But that doesn't mean there weren't "code talkers" in Europe. The army just used alternate languages. There were 13 Comanche code talkers assigned to the 4th division on D-day for example. And the army used several other languages including other Native American languages and Basque for some reason.
The Comanche Code Talkers of World War II, by William C. Meadows covers the Comanche talkers quite well, and touches on the other code talkers including the Choctaw Code Talkers of WW1.
Did the Germans and Japanese realize the codes were in native american languages?
Amateur historian chipping in! The British used Welsh in a similar way the Americans used Navajo. So I guess there were equivalents.