When using QDoubleValidator to validate QLineEdit input I take notice that it is possible to use both "C" locale and system locale to identificate what groupSeparator and decimalPoint can be used.

Step by step:

• I don't specify QLocale for QDoubleValidator, so used system locale by default
• I'm expect for ' ' (whitespace) as groupSeparator and ',' (comma) as decimalPoint (for my Locale RU_ru)
• When I'm run code (see below) I can use groupSeparator ',' and decimalPoint '.' (that is "C" locale) or I can use groupSeparator ' ' and decimalPoint ',' (that is my system locale) instead

I'm assume that only system locale must be available for QDoubleValidator. Otherwise user may be confused, what means ',' (comma) - groupSeparator or decimalPoint.

This code can help to reproduce my situation:

#include <QApplication>
#include <QLineEdit>
#include <QDoubleValidator>
#include <QLocale>
#include <QDebug>

int main(int argc, char *argv[])
{
	QApplication app(argc, argv);

	QDoubleValidator validator;
	validator.setLocale(QLocale::system());

	qDebug() <<
			"language: " <<
			QLocale::languageToString(validator.locale().language()) << "\n" <<
			"country: " <<
			QLocale::countryToString(validator.locale().country());

	QLineEdit edtTest;
	edtTest.setValidator(&validator);

	edtTest.show();
	return app.exec();
}

Try to enter "123,123,123.123".
And then delete it and try to enter "123 123 123,123".