Foreign “code talkers” were used in the Department of the West under the command of General John C. Fremont to transmit messages. What language was used to prevent the Confederates from deciphering the code?
The dialect of the Delaware Indians. "General John Fremont alsoexpressed interest in Indian enlistment. He took a particular interest in the Delaware Indians, andhe wished to enlist the aid of the Delaware warrior Falleaf (or Fall Leaf). As Hauptman notes,Falleaf was “[a] longtime associate of the famous explorer...who frequently hired the Delawarefor his majortopographical surveys of the American West....”92Fremont asked the warrior torecruit a company of his tribesmen and proceed to Springfield, Missouri, on a special mission.Falleaf was able to recruit fifty-four Delawares, and he was able to guide them fromSedalia toSpringfield without the Confederates spotting him.93Abel adds some more information to thisstory. She notes that the chiefs of the Delaware tribe refused their assent, but Falleaf and fifty-three of his fellow Delawares joined nonetheless. Fremont employed these Delawares as scouts and guides "
Edit - The source says the Delawares were employed as scouts and guides, but doesn't say they were employed as code talkers.
Hungarian was successfully employed via telegram and written letter. [Papers of US Grant vol.2, pages 178-200] Sending messages in Hungarian was easier than using code or cypher and considered safer. Even if the message was intercepted, there were few Confederate Hungarians, so little concern that it would be deciphered. http://suvcw.org/mollus/art014.htm
I don't think the idea of using the Navajo language as a military code was thought of until the 20th century.
Fremont had a force of personal bodyguards commanded by the Hungarian Charles Zagonyi. My guess is that his code talkers came from the ranks of those personal bodyguards and the language they used was Hungarian.
Because of the number of Hungarians serving on his staff and in various units attached to the Western Department, Fremont sent many of his communiques, particularly sensitive ones, to his subordinate commanders in Hungarian. Sending messages in Hungarian was simpler and safer than using some type of code. Even if intercepted, the message was unlikely to be "deciphered" because there were very few Hungarian-speakers on the Confederate side.”