add_action( 'init', 'build_taxonomies', 0 );  

function build_taxonomies() {
  register_taxonomy( 'category', 'post', array(
		'hierarchical' => true,
	 	'update_count_callback' => '_update_post_term_count',
		'query_var' => 'category_name',
		'rewrite' => did_action( 'init' ) ? array(
					'hierarchical' => false,
					'slug' => get_option('category_base') ? get_option('category_base') : 'category',
					'with_front' => false) : false,
		'public' => true,
		'show_ui' => true,
		'_builtin' => true,
	) );
}

add_filter( 'post_link', 'remove_parent_cats_from_link', 10, 3 );
function remove_parent_cats_from_link( $permalink, $post, $leavename )
{
    $cats = get_the_category( $post->ID );
    if ( $cats ) {
        // Make sure we use the same start cat as the permalink generator
        usort( $cats, '_usort_terms_by_ID' ); // order by ID
        $category = $cats[0]->slug;
        if ( $parent = $cats[0]->parent ) {
            // If there are parent categories, collect them and replace them in the link
            $parentcats = get_category_parents( $parent, false, '/', true );
            // str_replace() is not the best solution if you can have duplicates:
            // myexamplesite.com/luxemburg/luxemburg/ will be stripped down to myexamplesite.com/
            // But if you don't expect that, it should work
            $permalink = str_replace( $parentcats, '', $permalink );
        }
    }
    return $permalink;
}