What you need is Shortcode API.
Just add your own shortcode (put this into your themes functions.php or as a plugin):
function my_shortcode_xcode_handler( $atts, $content = null ) {
return '<pre class="lang:default decode:true ">'. do_shortcode($content) .'</pre>';
}
add_shortcode( 'xcode', 'my_shortcode_xcode_handler' );