Icons in select

I followed an example of cpen or something like that, I do not remember what the page is called and my page does not show the images I need. This is the example: https://codepen.io/brapastor/pen/BWyomB

these are the codes:

home.ctp

<select class="selectpicker" data-width="fit">
                <option  data-content='<span class="flag-icon flag-icon-mx" value="es"></span> Español'>Español</option>
                <option data-content='<span class="flag-icon flag-icon-us" value="en"></span> English'>English</option>
            </select>

js

  $(document).ready(function () {

        $(function () {
            $('.selectpicker').selectpicker();
        });
...

default.ctp

   echo $this->Html->css('bootstrap.min');
   echo $this->Html->css('bootstrap-datepicker3');
   echo $this->Html->css('https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.6.2/css/bootstrap-select.min');
   echo $this->Html->css('https://cdnjs.cloudflare.com/ajax/libs/flag-icon-css/0.8.2/css/flag-icon.min');

   echo $this->Html->script('jquery-3.3.1');
   echo $this->Html->script('bootstrap.min');
   echo $this->Html->script('https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.6.2/js/bootstrap-select.min');
   echo $this->Html->script('https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.6.2/js/bootstrap-select.min');

in case it serves as something I use cakephp 2.x