I don't see any other way then to have an array containing every word, from zero to one hundred.
var words:Array = [zero, one, two, ...];
Then, when you listen to the progress event, you would Math.round the result of bytes percentage and show words[result] instead of the actual number.
While it might be a novel approach, it may not be a practical one. When you use numbers, there's is minimal shifting of readable information. When you use the words of the numbers, things can start jumping all over the place, making the reading of anything unlikely, especially if the loading time is short.