How to customize the "Remove" button in a multiple field (collection)
The solution is easy, but it took me a couple of hours wading through Drupal Core nevertheless:

foreach($form['field_termine']['und'] as $k=&gt;$v) {
  if(!is_numeric($k)) //we only want the individual items...
    continue;
  $form['field_termine']['und'][$k]['remove_button']['#value']="Kill";
}


Note that you will get in trouble with multi-language support with this, YMMV.