when I had
$wgExtensionMessagesFiles['Purge'] = $dir . 'Purge.i18n.php';
in Purge.php
my http://projetgentilsvirus.ouvaton.org/index.php/Sp%C3%A9cial:Version was empty ... don't know why ... probably because of another extension ??
I just commented it and had sthg like :
'text' => 'Actualiser - Refresh' , // wfMsg( 'purge' ),
so that the purge label is OK on my wiki