so when you passing "12345.67" this then it should return "12 347,67"

But you are asserting "-12" + separator + "345,67 "

I tested locally and it works as expected

@Test

fun test(){

val separator = DecimalFormatSymbols.getInstance(Locale.CANADA_FRENCH).groupingSeparator

val formatted = NumberFormat.getInstance(Locale.CANADA_FRENCH).format(12345.67)

assertEquals("12${separator}345,67", formatted)

}