How to convert XML from Latin 2 to UTF 8 in Android/C#/Xamarin

I have a problem. In my app i download XML file from the Web. Its encoding is ISO-8859-2. I put some data from this XML to Spinner and there are questions marks instead of some polish characters. Have I to change encoding to UTF-8 to solve that problem? If yes, how to do it? I tried using method found on the Internet but it doesn't help

private XmlDocument ChangeXmlEncoding(XmlDocument xmlDoc, string newEncoding)
    {
        if (xmlDoc.FirstChild.NodeType == XmlNodeType.XmlDeclaration)
        {
            XmlDeclaration xmlDeclaration = (XmlDeclaration)xmlDoc.FirstChild;
            xmlDeclaration.Encoding = newEncoding;
        }
        return xmlDoc;
    }

public static string GetPageAsString(System.Uri address)  
    {  
        string result = "";  

        // Create the web request  
        HttpWebRequest request = WebRequest.Create(address) as HttpWebRequest;  

        // Get response  
        using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)  
        {  
            // Get the response stream  
            StreamReader reader = new StreamReader(response.GetResponseStream());  


            // Read the whole contents and return as a string  
            result = reader.ReadToEnd();  
        }  

        return result;  
    } 

and usage:

                System.Uri uri = new System.Uri("http://nbp.pl/kursy/xml/LastA.xml");
                XMLString = GetPageAsString(uri);
                dokument = new XmlDocument ();
                dokument.LoadXml(XMLString);
                dokument = ChangeXmlEncoding(dokument,"utf-8");