The decimal separator is tied to your language it seems. A workaround is suggested by Pieter and Arjen here.
Sorry Pieter, Arjan and Samet, your solutions is more a 'duck-tape' solution. When starting in dutch, everything is OK, but when you start your project in language US and add Dutch later on. The numeric separators will be US based. Even for those account who has Dutch as their language. So Mx, please add a setting like you did for dates