using System;
using System.Collections;
using System.Collections.Generic;
using System.Net.Http;					// add NuGet package
using Newtonsoft.Json;					// add NuGet package

public class Program
{
      public static void Main()
      {
	 // translate "hello world" from english to french
	 Console.WriteLine(TranslateText("hello world", "en", "fr"));
      }
	
      public static string TranslateText(string input, string langCdOriginal, string langCdTranslation)
      {
         string url = String.Format
         ("https://translate.googleapis.com/translate_a/single?client=gtx&sl={0}&tl={1}&dt=t&q={2}",
         langCdOriginal, langCdTranslation, Uri.EscapeUriString(input));
		
         HttpClient httpClient = new HttpClient();
         string result = httpClient.GetStringAsync(url).Result;

         // Get all json data
         var jsonData = JsonConvert.DeserializeObject<List<dynamic>>(result);

         // Extract just the first array element (This is the only data we are interested in)
         var translationItems = jsonData[0];

         // Translation Data
         string translation = "";

         // Loop through the collection extracting the translated objects
         foreach (object item in translationItems)
         {
            // Convert the item array to IEnumerable
            IEnumerable translationLineObject = item as IEnumerable;

            // Convert the IEnumerable translationLineObject to a IEnumerator
            IEnumerator translationLineString = translationLineObject.GetEnumerator();

            // Get first object in IEnumerator
            translationLineString.MoveNext();

            // Save its value (translated text)
            translation += string.Format(" {0}", Convert.ToString(translationLineString.Current));
         }

         // Remove first blank character
         if (translation.Length > 1) { translation = translation.Substring(1); };

         // Return translation
         return translation;
    }
}

Unfortunately by using the free Google Translate API you will only be allowed to translate a certain ammount of words per hour. After this, Google API will return a 429 (too many requests) error.