add span to wp_nav_menu widget

Filter 'walker_nav_menu_start_el' and replace the link text.

Prototype, not tested:

add_filter( 'walker_nav_menu_start_el', 'wpse_56028_title' );

function wpse_56028_title( $item )
{
    return preg_replace( '~(<a[^>]*>)([^<]*)</a>~', '$1<span>$2</span></a>', $item);
}