public class IndexTest 
    {
        public IndexTest() 
		{
			_list.Add("foobar");
			_dict.Add("foo", "bar");
		}
		
        private List<string> _list = new List<string>();
        private Dictionary<string, string> _dict = new Dictionary<string, string>();
        private Lexicon _lex = new Lexicon();

        public List<string> List { get { return _list; } }
        public Dictionary<string, string> Dict { get { return _dict;  } }
        public Lexicon Lex { get { return _lex; } }

        //Custom class with indexer.
        public class Lexicon 
	    {
		    public string this[string arg] 
		    {
                get 
                {
                    var str = "foo " + arg;
                    return str; 
                }
		    }
	    }
    }

<StackPanel>
		<TextBlock Text="{Binding List[0]}"/>
		<TextBlock Text="{Binding Dict[foo]}"/>
		<TextBlock Text="{Binding Lex[foo]}"/> 
	</StackPanel>

//NotifyPropertyChangedBase is simply an implementation of INotifyPropertyChanged
public class Lexicon : NotifyPropertyChangedBase, IDictionary<string, string>
{

    //This is the backend class I am writing a wapper for.
    private LanguageManager _mgr { get { return LanguageManager.Instance; } }
	
	public SmartCultureInfo Culture 
	{
		set 
        {
            Debug.Log("New culture: " + value.nativeName);
            _mgr.ChangeLanguage(value);
            NotifyPropertyChanged("Item[]");
        }
		get 
        {
            var mgr = _mgr;
            return mgr.GetCultureInfo(mgr.LoadedLanguage);
        }
	}

    public List<SmartCultureInfo> Languages
    {
        get { return _mgr.GetSupportedLanguages(); }
    }

    #region IDictionary
    public bool ContainsKey(string key)
    {
        var found = _mgr.HasKey(key);
        Debug.Log("Looking for key: " + key + " found: " + found );
        return found;
    }

    public string this[string key]
    {
        get
        {
            Debug.Log("Fetch key: " + key);
            return _mgr.GetTextValue(key);
        }
        set
        {
            Debug.Log("Calling set on indexer for key: " + key);
            throw new System.NotSupportedException();
        }
    }

    public bool Contains(KeyValuePair<string, string> item)
    {
        Debug.Log("Contains?");
        var val = _mgr.GetTextValue(item.Key);
        var contains = item.Value.Equals(val);
        return contains;
    }
    
    #endregion

    #region Not supported features

    

    public bool TryGetValue(string key, out string value)
    {
        Debug.Log("Called TryGetValue");
        throw new System.NotSupportedException();
    }

    public ICollection<string> Values
    {

        get { Debug.Log("Called Values"); throw new System.NotSupportedException(); }
    }

    public void Add(string key, string value)
    {
        Debug.Log("Called Add");
        throw new System.NotSupportedException();
    }
    public bool Remove(string key)
    {
        Debug.Log("Called Remove");
        throw new System.NotSupportedException();
    }
    public ICollection<string> Keys
    {
        get { Debug.Log("Called Keys"); throw new System.NotSupportedException(); }
    }
    public System.Collections.IEnumerator GetEnumerator()
    {
        Debug.Log("Called GetEnumerator");
        throw new System.NotSupportedException();
    }

    public void Add(KeyValuePair<string, string> item)
    {
        Debug.Log("Called Add");
        throw new System.NotSupportedException();
    }

    public void Clear()
    {
        Debug.Log("Called Clear");
        throw new System.NotSupportedException();
    }
    public void CopyTo(KeyValuePair<string, string>[] array, int arrayIndex)
    {
        Debug.Log("Called CopyTo");
        throw new System.NotSupportedException();
    }

    public int Count
    {
        get { Debug.Log("Called Count"); throw new System.NotSupportedException(); }
    }

    public bool IsReadOnly
    {
        get { Debug.Log("Called IsReadOnly"); throw new System.NotSupportedException(); }
    }

    public bool Remove(KeyValuePair<string, string> item)
    {
        Debug.Log("Called Remove");
        throw new System.NotSupportedException();
    }

    IEnumerator<KeyValuePair<string, string>> IEnumerable<KeyValuePair<string, string>>.GetEnumerator()
    {
        Debug.Log("Called GetEnumerator2");
        throw new System.NotSupportedException();
    }

    #endregion

}

<TextBlock Text="{Binding Lex[LeftBox]}"/>
<TextBlock Text="{Binding Lex[RightBox]}"/>
<ComboBox ItemsSource="{Binding Lex.Languages}" SelectedItem="{Binding Lex.Culture}"/>