using System;

​

public class Program

{

    public static void Main()

    {

        var germanCulture = System.Globalization.CultureInfo.GetCultureInfo("de-de");

        decimal sourceValue = 0.123m;

        string target_invariant = sourceValue.ToString(System.Globalization.CultureInfo.InvariantCulture);

        string targetValue = sourceValue.ToString(germanCulture);

        Console.WriteLine(target_invariant);

        Console.WriteLine(targetValue);     

    }

}