So you need to use a rw_display_widget filter hook here and WP conditional tags (http://codex.wordpress.org/Conditional_Tags).
For example:
function custom_rw_display_widget($display, $instance) {
// instance title is the widget title, in category can by category name, slug, ID or array of IDs
if ($instance['title'] == 'Partir en Voyage' && in_category('Afrique du Sud')) {
// display or hide the widget
$display = true; // or false
}
return $display;
}
add_filter('rw_display_widget', 'custom_rw_display_widget')