Query for custom post type objects in a taxonomy and with a meta value

Since in your comment, you said that you just want posts with meta key lang=en on a custom taxonomy page, the easiest way to do that would be to filter the query with pre_get_posts before it is run.

function wpa_107371_meta_query( $query ) {
    if ( is_admin() || ! $query->is_main_query() )
        return;

    // only change the query on a custom taxonomy
    // can check for a specific taxonomy if desired
    if ( is_tax() ) {
        //define our meta query
              $meta_query = array(
                  array(           // needs this nested array syntax to work
                    'key'    => 'lang',
                    'value'  => 'en',
                    'compare'=> '=',
                  ),
              );
         $query->set('meta_query', $meta_query);
        return;
    }

}
add_action( 'pre_get_posts', 'wpa_107371_meta_query' );