How does French qualify as using a base-20 counting system? Intuitively, I'd expect that to mean

- special words for the numbers 1-19

- special words for 20, 40, 60, 80, 100, 120, ..., 380

- special words for 400, 8000, 160,000, etc.

Instead, what we actually see in French is

- special words for the numbers 1-16

- special words for 20, 30, 40, 50, 60, and 80

- special words for 100, 1000, 1,000,000, etc.

This is a base-10 system (with the odd quirk), as far as I can see.