Hi Santiago,

You can use python code to convert number to words.

install num2words python package in your system

          from num2words import num2words
          num2words(Subtotal, lang='es') //here es for Spanish

Or you can refer

https://stackoverflow.com/questions/13913165/python-script-to-convert-from-numbers-to-words-for-printing-cheques/13932483
      

Write below code in your .js file

      var _super_order = models.Order.prototype;
      models.Order = models.Order.extend({
      export_for_printing: function() {
          var json =
      _super_order.export_for_printing.apply(this,arguments);
          json.subtotal_in_words = this.get_subtotal_in_words();
          return json;
          },
      get_subtotal_in_words:function(){
          var subtotal = this.get_total_without_tax();
          //here call your python code to convert subtotal to words and return it OR you can use js code to convert subototal into words
 },
      });

now to print this in receipt write below code in xml:

      <t t-esc="widget.format_currency(order.get_subtotal_in_words())"/>

Hope this will help you!

Thanks.