class StringFormatConverter : IMultiValueConverter
{
    Dictionary<int, Tuple<int, object, IMultiValueConverter>> nestedConverters = new Dictionary<int, Tuple<int, object, IMultiValueConverter>>();
    public void AddNestedConverter(IMultiValueConverter c, object parameter, int fromidx, int count)
    {
        nestedConverters.Add(fromidx, Tuple.Create(count, parameter, c));
    }

    List<object> Simplify(object[] values, CultureInfo culture)
    {
        List<object> result = new List<object>();

        for (int i = 0; i < values.Length; /**/)
        {
            object curr;
            Tuple<int, object, IMultiValueConverter> nested;
            if (nestedConverters.TryGetValue(i, out nested))
            {
                var nestedConverter = nested.Item3;
                var parameter = nested.Item2;
                var length = nested.Item1;
                var nestedValues = new object[length];
                Array.Copy(values, i, nestedValues, 0, length);
                curr = nestedConverter.Convert(nestedValues, typeof(object), parameter, culture);
                i += length;
            }
            else
            {
                curr = values[i];
                i++;
            }
            result.Add(curr);
        }

        return result;
    }

    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        var simplifiedValues = Simplify(values, culture);
        var format = (string)simplifiedValues[0];
        return string.Format(format, simplifiedValues.Skip(1).ToArray());
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}