Conversation so far:
What was the Olmec people's ethnicity?
While the actual ethno-linguistic affiliation of the Olmec remains unknown, various hypotheses have been put forward.
What was the Olmec people's language?
In 1976, linguists Lyle Campbell and Terrence Kaufman proposed that the Olmec spoke a language ancestral to Mixe–Zoquean.

Question: What else do we know about their language?
Self-contained rewrite: What else do we know about the Olmec's language besides that it is a language ancestral to Mixe-Zoquean.