これを試して
Dictionary<string,string> objDic = new Dictionary<string,string>();
foreach (CultureInfo ObjCultureInfo in CultureInfo.GetCultures(CultureTypes.SpecificCultures))
{
RegionInfo objRegionInfo = new RegionInfo(ObjCultureInfo.Name);
if (!objDic.ContainsKey(objRegionInfo.EnglishName))
  {
      objDic.Add(objRegionInfo.EnglishName, objRegionInfo.TwoLetterISORegionName.ToLower());
  }
}
var obj = objDic.OrderBy(p => p.Key );
foreach (KeyValuePair<string,string> val in obj)
{
  ddlCountries.Items.Add(new ListItem(val.Key, val.Value));
}
EnglishName国名を返します
IPから見る