It's true that when we talk about an excerpt, nobody understands what it refers to.
To change the title of this editing area, just use the following snippet of code:
add_filter( 'gettext', 'wpse22764_gettext', 10, 2 );
function wpse22764_gettext( $translation, $original )
{
if ( 'Excerpt' == $original ) {
return 'Sous-titre';
}else{
$pos = strpos($original, 'Excerpts are optional hand-crafted summaries of your');
if ($pos !== false) {
return 'Sous-titre affiché dans les push.';
}
}
return $translation;
}