CSS/JS is not working in multi lang installation

Enqueue the files like they should be enqueued and HTTP(S) won’t be a problem.

You can enqueue files properly witht he following example code

/**
 * Enqueue scripts
 *
 * @param string $handle Script name
 * @param string $src Script url
 * @param array $deps (optional) Array of script names on which this script depends
 * @param string|bool $ver (optional) Script version (used for cache busting), set to null to disable
 * @param bool $in_footer (optional) Whether to enqueue the script before </head> or before </body>
 */
function theme_name_scripts() {
    wp_enqueue_script( '$handle', '$src', array( 'jquery' ), false, false);
    wp_enqueue_style( $handle, $src, $deps, $ver, $media );
}

add_action( 'wp_enqueue_scripts', 'theme_name_scripts' );