GetCountryList.cs

using System.Collections.Generic;
using System.Globalization;

public static IEnumerable<string> GetCountryList()
{
    List<string> cultureList = new List<string>();

    CultureInfo[] cultures = CultureInfo.GetCultures(CultureTypes.SpecificCultures);

    foreach (CultureInfo culture in cultures)
    {
        RegionInfo region = new RegionInfo(culture.LCID);

        if (!(cultureList.Contains(region.EnglishName)))
        {
            cultureList.Add(region.EnglishName);
        }
    }
    return cultureList;
}