Bad Request - Request Too Long
HTTP Error 400. The size of the request headers is too long.

http://ajax.googleapis.com/ajax/services/language/translate?v=1.0&q={0}&langpair={1}|{2}

//{\"responseData\": {\"translatedText\":\"سلام جهان!\"}, \"responseDetails\": null, \"responseStatus\": 200}

      GoogleAjaxResponse result =
          new JavaScriptSerializer().Deserialize<GoogleAjaxResponse>(jsonGoogleAjaxResponse);

//ResponseData.cs file
public class ResponseData
{
  public string translatedText { get; set; }
}

//GoogleAjaxResponse.cs file
using System.Net;

/// <summary>
/// کلاسی جهت نگاشت اطلاعات جی سون دریافتی به آن
/// </summary>
public class GoogleAjaxResponse
{
  public ResponseData responseData { get; set; }
  public object responseDetails { get; set; }
  public HttpStatusCode responseStatus { get; set; }
}

using System;
using System.Globalization;
using System.IO;
using System.Net;
using System.Web;
using System.Web.Script.Serialization;

//{\"responseData\": {\"translatedText\":\"سلام جهان!\"}, \"responseDetails\": null, \"responseStatus\": 200}

public class CGoogleTranslator
{
  #region Fields (1)

  /// <summary>
  /// ارجاع دهنده
  /// </summary>
  private readonly string _referrer;

  #endregion Fields

  #region Constructors (1)

  /// <summary>
  /// مطابق مستندات نیاز به یک ارجاع دهنده اجباری می‌باشد
  /// </summary>
  /// <param name="referrer"></param>
  public CGoogleTranslator(string referrer)
  {
      _referrer = referrer;
  }

  #endregion Constructors

  #region Properties (2)

  /// <summary>
  /// ترجمه از زبان
  /// </summary>
  public CultureInfo FromLanguage { get; set; }

  /// <summary>
  /// ترجمه به زبان
  /// </summary>
  public CultureInfo ToLanguage { get; set; }

  #endregion Properties

  #region Methods (2)

  // Public Methods (1)

  /// <summary>
  /// ترجمه متن با استفاده از موتور ترجمه گوگل
  /// </summary>
  /// <param name="data"></param>
  /// <returns></returns>
  public string TranslateText(string data)
  {
      //ساخت و انکدینگ آدرس مورد نظر
      string url =
          string.Format(
             "http://ajax.googleapis.com/ajax/services/language/translate?v=1.0&q={0}&langpair={1}|{2}",
             HttpUtility.UrlEncode(data), //needs a ref. to System.Web.dll
          FromLanguage.TwoLetterISOLanguageName,
          ToLanguage.TwoLetterISOLanguageName
      );

      //دریافت اطلاعات جی سون از گوگل
      string jsonGoogleAjaxResponse = fetchWebPage(url);

      //needs a ref. to System.Web.Extensions.dll
      //نگاشت اطلاعات جی سون دریافت شده به کلاس مرتبط
      GoogleAjaxResponse result =
          new JavaScriptSerializer().Deserialize<GoogleAjaxResponse>(jsonGoogleAjaxResponse);

      if (result != null && result.responseData != null && result.responseStatus == HttpStatusCode.OK)
      {
          return result.responseData.translatedText;
      }
      return string.Empty;
  }
  // Private Methods (1)

  /// <summary>
  /// دریافت محتویات جی سون بازگشتی از گوگل
  /// </summary>
  /// <param name="url"></param>
  /// <returns></returns>
  string fetchWebPage(string url)
  {
      try
      {
          var uri = new Uri(url);
          if (uri.Scheme == Uri.UriSchemeHttp || uri.Scheme == Uri.UriSchemeHttps)
          {
              var request = WebRequest.Create(uri) as HttpWebRequest;
              if (request != null)
              {
                  request.Method = WebRequestMethods.Http.Get;
                  request.Referer = _referrer;
                  request.UserAgent = "Mozilla/5.0 (Windows; U; Windows NT 5.0; ; rv:1.8.0.7) Gecko/20060917 Firefox/1.9.0.1";
                  request.AllowAutoRedirect = true;
                  request.Timeout = 1000 * 300;
                  request.KeepAlive = false;
                  request.ReadWriteTimeout = 1000 * 300;
                  request.AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate;

                  using (var response = request.GetResponse() as HttpWebResponse)
                  {
                      if (response != null)
                      {
                          using (var reader = new StreamReader(response.GetResponseStream()))
                          {
                              return reader.ReadToEnd().Trim();
                          }
                      }
                  }
              }
          }
          return string.Empty;
      }
      catch (Exception ex)
      {
          Console.WriteLine(String.Format("fetchWebPage: {0} >> {1}", ex.Message, url), true);
          return string.Empty;
      }
  }

  #endregion Methods
}

string res = new CGoogleTranslator("https://www.dntips.ir/")
  {
     FromLanguage = CultureInfo.GetCultureInfo("en-US"),
     ToLanguage = CultureInfo.GetCultureInfo("fa-IR")
   }.TranslateText("Hello world!");