Main.cs

using System;
using System.Globalization;

public static class TimeSpanExtensions
{
    public static string LocalizedTimeFormat(
        this TimeSpan timeSpan, CultureInfo cultureInfo)
    {
        string formattedTimeSpan = timeSpan.ToString(@"hh\:mm\:ss");
        
        string timeSeparator = cultureInfo.DateTimeFormat.TimeSeparator;

        return formattedTimeSpan.Replace(":", timeSeparator);
    }
}

class MainClass
{
    public static void Main (string[] args)
    {
        var dateTime1 = new DateTime(1970, 1, 1, 0, 0, 0);
        var dateTime2 = new DateTime(2020, 4, 1, 13, 30, 22, 500);

        TimeSpan span = dateTime2 - dateTime1;

        Console.WriteLine(span.LocalizedTimeFormat(new CultureInfo("en-US")));
        // => 13:30:22

        Console.WriteLine(span.LocalizedTimeFormat(new CultureInfo("fr-FR")));
        // => 13:30:22

        Console.WriteLine(span.LocalizedTimeFormat(new CultureInfo("ml-IN")));
        // => 13.30.22
    }
}