您可以使用gettext 过滤器：

/**
 * Rename the \'Username\' label in wp-login.php
 *  
 * @see http://wordpress.stackexchange.com/a/159507/26350
 */

add_filter( \'wp_login_errors\',
    function( $errors )
    {
        add_filter( \'gettext\', \'wpse_change_username\', 99, 3 );
        return $errors;
    }
);

function wpse_change_username( $translated_text, $untranslated_text, $domain )
{
    $old = "Username";
    $new = "Jediname";

    if ( $untranslated_text === $old )
        $translated_text = $new;

    remove_filter( current_filter(), __FUNCTION__ );

    return $translated_text;
}

以下是屏幕截图：