How to alter cck field title using hook_form_alter?
First, you may check the exact field array with dsm() 

function MODULE_form_taxonomy_form_term_alter(&amp;$form, &amp;$form_state, $form_id) {
  dsm($form);
}




In the result returned by dsm(),  #title is set in 2 places. So just try :

$form['field_bibliographie']['#title']= t('Foo');//doesn't work
$form['field_bibliographie']['und'][0]['#title']= t('Foo');//correct one


I see no reason for which it would not take the correct value if you set the exact field array