Magic Keyboard with number pad: type comma instead of a dot

System Preferences- Language and Region- at the bottom of the box click "Advanced"- then choose the dot/ comma under Number separators- Decimal and again under drop down box "decimal" under currency.

I know it is an old question, but for those like me who changed decimals to comma just to find out it did nothing, you can use this approach.

hidutil property --set '{"UserKeyMapping": [{"HIDKeyboardModifierMappingSrc": 0x700000063, "HIDKeyboardModifierMappingDst": 0x700000036}]}'

https://developer.apple.com/library/archive/technotes/tn2450/_index.html

It replaces the , from the keypad to . It can be reverted changing the values 0x7... but anyway, IMO comma should be the key by default, as it is the most common decimal separator. https://en.wikipedia.org/wiki/Decimal_separator#/media/File:DecimalSeparator.svg