C#: Custom number formatting in rigth to left languages

I am trying to write a number with a custom formatting in a right-to-left textbox(actually in a listview but whatever). here is the code:

NumberFormatInfo nfi = (NumberFormatInfo)
            CultureInfo.InvariantCulture.NumberFormat.Clone();
nfi.NumberGroupSeparator = " ";
textbox1.Text = (18700).ToString("#,###",nfi);

instead of 18 700 i get 700 18. can i fix this(get the required 18 700) without manipulating the string after number to string conversion?