Complex sentence: Since Nahuatl (Nāhuatl) (Aztecs'language) didn't have a writing system, when Spanish conquistadors arrived, they wrote the language with their own alphabet without distinction on long vowels.
Split into simpler sentences: Since Nahuatl (Nāhuatl) (Aztecs' language) did not have a writing system. When Spanish conquistadors arrived, they wrote the language with their own alphabet without distinguishing long vowels.