Complex sentence: The development of an orthography in the late 1930s helped spread education among Navajos, but the language was still dwindling in use: by the 1960s, indigenous languages of the United States had been declining in use for some time.
Split into simpler sentences: The development of an orthography in the late 1930s helped spread education among Navajos. However, the language was still dwindling in use: by the 1960s, indigenous languages of the United States had been declining in use for some time.