Going by the Wikipedia article and included map, use of comma versus period as decimal separators is roughly an even split:
https://en.wikipedia.org/wiki/Decimal_separator
https://commons.wikimedia.org/wiki/File:DecimalSeparator.svg
https://en.wikipedia.org/wiki/Decimal_separator
https://commons.wikimedia.org/wiki/File:DecimalSeparator.svg
Maybe someone with a CSV of the world populations and a CSV of the countries broken down by their separator can do that comparison.