Nette\Forms\Controls\SelectBox - <optgroup>

nette-select-optgroup.php

<?php

$countries = array(
    'eu' => array(
        'cs'    => 'Cesko',
        'sk'    => 'Slovensko',
    ),
    'en'    => 'USA'
);

$form->addSelect('country', 'Zeme:', $countries);


?>

<select name="country" id="frm-myForm-country">
  <optgroup label="eu">
    <option value="cs">Cesko</option>
    <option value="sk">Slovensko</option>
  </optgroup>
  <option value="en">USA</option>
</select>