From the device you can only get the language, if you want to get the region code, you will need to get the user location using geolocator https://pub.dev/packages/geolocator then you can use geocoding https://pub.dev/packages/geocoding to get code from coordinates.

Get Location

Future<Position> getCurrentLocation(BuildContext context) async {
  bool serviceEnabled;
  LocationPermission permission;

  serviceEnabled = await Geolocator.isLocationServiceEnabled();
  if (!serviceEnabled) {
    return Future.error('Location services are disabled.');
  }

  permission = await Geolocator.checkPermission();
  if (permission == LocationPermission.deniedForever) {
    return Future.error(
        'Location permissions are permanently denied, we cannot request permissions.');
  }

  if (permission == LocationPermission.denied) {

    permission = await Geolocator.requestPermission();
    if (permission != LocationPermission.whileInUse &&
        permission != LocationPermission.always) {
      return Future.error(
          'Location permissions are denied (actual value: $permission).');
    }
  }

  return await Geolocator.getCurrentPosition();
}

Decode location using Geocoding

position = await getCurrentLocation(context);
placeMarks = await placemarkFromCoordinates(position.latitude, position.longitude);
if (placeMarks.isNotEmpty) {
    debugPrint('=======> ${placeMarks.first.isoCountryCode}');
}

It will show MM for Myanmar.