NumberFormatter has several different configuration properties, eg decimal and thousands separator character. I don't think there is any kind of property you can use just to set a localisation parameter, you have to do each one separately. You can set this up either in MXML eg
<mxml:NumberFormatter decimalSeparatorTo="," />
or in actionScript at run time eg
var nf: NumberFormatter = new NumberFormatter();
nf.decimalSeparatorTo = ","