I was using .htaccess to redirect multilingual content to virtual folders,

I had registered sitemap.xml for each language with Google, but it started throwing errors.

I monitored it for a while, but since it stopped appearing in search results, I took countermeasures.

I moved the configuration from .htaccess to WordPress's function.php and added the following entries:

// Add virtual URLs for language codes

function add_lang_rewrite_rules() {

    add_rewrite_rule(

        '^(ja|en|ko|cn|tw)/?$',

        'index.php?lang=$matches[1]',

        'top'

    );

    add_rewrite_rule(

        '^(ja|en|ko|cn|tw)/(.*)/?$',

        'index.php?lang=$matches[1]&pagename=$matches[2]',

        'top'

    );

}

add_action('init', 'add_lang_rewrite_rules');

// Add lang to query variables

function add_lang_query_var($vars) {

    $vars[] = 'lang';

    return $vars;

}

add_filter('query_vars', 'add_lang_query_var');

function get_current_lang() {

    $lang = get_query_var('lang');

    if (!$lang) {

        return 'ja'; // Default language

    }

    return $lang;

}

add_action('template_redirect', function() {

    if (is_admin()) return;

    $request_uri = trim(parse_url($_SERVER['REQUEST_URI'], PHP_URL_PATH), '/');

    // Exclude sitemap.xml, robots.txt, CSS/JS/images, etc.

    if (preg_match('/.(xml|txt|css|js|png|jpg|jpeg|gif)$/i', $request_uri)) {

        return;

    }

    // Redirect only the top page / to /ja/

    if ($request_uri === '' || $request_uri === 'index.php') {

        wp_redirect(home_url('/ja/'), 301);

        exit;

    }

});

We'll continue monitoring the situation for a while longer.

We are using the site below for our sitemap

Create Sitemap - Automatic Generation Tool "sitemap.xml Editor"