Believe it or not, the hardest is English! The reason being that it has so many words that sound alike but spelled different and so many words spelled alike but pronounced different.like read.[reed,red]Then there are the words pronounced and spelled alike but have different meanings. Very confusing to learn!
Navajo, it was used by the military to send coded messages, they were known as "code talkers". Navajo is pretty much impossible to learn unless you grew up with the language, because so many different words have so many different meanings depending on the inflection put on the word.