Are you testing this on an Apple platform, or a different platform like Linux or Windows?
The source code you linked is only used on non-Apple platforms. Apple uses a private implementation of Foundation on its platforms. However, much of Swift layer on top of the private Foundation was part of the swift repo prior to release 5.5, including the implementation of Decimal(string:locale:):
public init?(string: __shared String, locale: __shared Locale? = nil) {
let scan = Scanner(string: string)
var theDecimal = Decimal()
scan.locale = locale
if !scan.scanDecimal(&theDecimal) {
return nil
}
self = theDecimal
}
Here we can see that, if you don't pass a Locale (or if you pass nil), it sets the scanner's locale to nil. The NSScanner documentation says
A scanner with no locale set uses non-localized values.
That is why your call to Decimal.init?(string:) always uses . as the decimal separator.