Note the keyboard part. I think this is meant to be what they can physically print on the numpad section of an international-style keyboard. When you're typing numbers in a Euro locale you'd get `,` but in US English you would get a period. (At least, I hope that's what the decimal key on the numpad does when you set your locale to a certain locale. It would be obnoxious if when typing numbers in an English context, you had to go find the period key on the other side.)

I don't think we're meant to normally switch to actually putting that character in our numbers though. If so, clearly nobody has agreed to do it!