Mesoamerican languages, in the Nahuatl and Maya families at least, are still spoken by millions of people. While the languages are not identical to older versions, such as those in Classic Maya inscriptions, the grammar is still readily recognizable, much as Spansh and Italian can be easily recognized as Latin descendants.

Nahuatl might not be a good example since it's still spoken. I know someone who speaks it. She corrected my pronunciation of the word Nahuatl. (The l in Nahuatl is pronounced similar to ll in Welsh, apparently)