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The code value of NumpadDecimal is good for i18n. However, the comma key on Numpad's code value is defined as NumpadComma. I think that NumpadSeparator is better for some countries' people.
This is another one of those keys that cannot have a name that matches every locale. This key is rare and for the few cases where it is present, we need to have a name that properly identifies it. Hence the names "Decimal" and "Comma", which are meant to go hand-in-hand to make the keys easy to identify for locales that swap the 2 meanings. In addition, the name "Separator" is very generic and might lead to confusion. FWIW, the USB spec, when faced with the same problem, chose "Keypad Comma" as the name for this key. See footnote 27 for Table 12 in the USB HID Usage Tables.
Indeed, the key is defined as "Comma" in USB spec. My concern is, one of the rare keyboard having this key is Brazilian keyboard (ABNT keyboard), but it's the most popular keyboard layout having the key. Unfortunately, Brazilian people use "." for thousand separator...