I've created dropdown menu with all current terms:

    $taxonomy_names = get_post_taxonomies( );

    $term = get_term_by( 'slug', get_query_var( 'term' ), get_query_var( 'taxonomy' ) ); 
    $term_name = $term->name;

    $args = array(
        'show_option_all'   => __( 'Wybierz kategorię', 'behold-basic' ),
        'show_count'       => 1,
        'orderby'           => 'name',
        'selected'           => 0,
        'depth'             => 3,
        'hierarchical'      => 1,
        'echo'               => 0,
        'selected'           => false,
        'orderby'          => 'name',
        'name'              => $term_name,
        'class'             => 'postform',
        'taxonomy'          => $taxonomy_names,
        'value_field'       => 'slug',
    );
        $select  = wp_dropdown_categories( $args );
        $replace = "<select$1 onchange='return this.form.submit()'>";
        $select  = preg_replace( '#<select([^>]*)>#', $replace, $select );
        echo $select;

In menu I see every term names, but when I choose one I get 404. For eg. if current term is "europe" URL is "travels/europe" and when I choose "africa", url is changed to "/?Europa=africa" instead of "travels/africa".

I'm using it in taxonomy.php file, one for all my CPT.