Code Talkers is an online exhibit from the National Museum of the American Indian. It tells about how Native Americans used their languages to help transmit messages as part of the United States military during World War II.
The site is very accessible to English Language Learners, with audio and text support, along with helpful images. Early Intermediate speakers and above should be able to understand much of it.
I’ve placed the link on my Geography & United States History page under Problems At Home and Across the Sea. You can also find it on my World History page in The World In the Twentieth Century section.