Seems geographically split, but I wonder what is the actual population split is. Most of the top 10 population countries use the decimal separator. Only Brazil, Russia and Indonesia don't.
Maybe someone with a CSV of the world populations and a CSV of the countries broken down by their separator can do that comparison.
https://en.wikipedia.org/wiki/Decimal_separator
https://commons.wikimedia.org/wiki/File:DecimalSeparator.svg