Alert a message after ajax callback
Add the ajax callback in the submit button of the form like this:

Edit

$form['radio_buttons']['#ajax'] = array (
  'callback' =&gt; '_test',
  'wrapper' =&gt; 'nimportequi',
  'event' =&gt; 'change',
  'progress' =&gt; array('type' =&gt; 'throbber', 'message' =&gt; ''),
);


This will ensure that when a radio button is clicked your ajax callback will be executed. Selecting radio buttons will give alert message, therefore the ajax callback should be applied to radio buttons.

Your ajax callback seems ok.