我明白了。缺少的是重新加载WooCommerce的文本域，该域在初始化时使用当前语言环境加载：

// set the current locale and send email with it active
unload_textdomain(\'woocommerce\');
setlocale(LC_ALL, $new_locale);
global $q_config, $locale, $woocommerce;
$locale = $new_locale;
$q_config[\'language\'] = substr($new_locale, 0, 2);
$woocommerce->load_plugin_textdomain();

global $wc_cle_wc_email;
$wc_cle_wc_email->customer_completed_order($order_id);

// set the old locale back
unload_textdomain(\'woocommerce\');
$q_config[\'language\'] = substr($old_locale, 0, 2);
$locale = $old_locale;
setlocale(LC_ALL, $old_locale);
$woocommerce->load_plugin_textdomain();

设置语言环境变量的一些调用可能是多余的和/或根本不必要的，但这是可行的。

好吧，对于那些后来偶然发现这个问题的人，这里有一个更完整的答案。

add_filter( \'woocommerce_checkout_fields\' , \'custom_override_checkout_fields\' );
function custom_override_checkout_fields( $fields ) {
         $fields[\'order\'][\'language\'][\'placeholder\'] = \'\';
         $fields[\'order\'][\'language\'][\'type\'] = \'text\';
         $fields[\'order\'][\'language\'][\'label\'] = \'language\';
         $fields[\'order\'][\'language\'][\'class\'] = array(\'hidden\'); 
         $fields[\'order\'][\'language\'][\'default\'] = qtrans_getLanguage();
     return $fields;
}

然后。。。您需要在签出时保存该字段，这很简单：

/**
 * Save the language to the order (despite what woocommerce doc says this is required)
 */
add_action( \'woocommerce_checkout_update_order_meta\', \'my_custom_checkout_field_update_order_meta\' );
function my_custom_checkout_field_update_order_meta( $order_id ) {
    if ( ! empty( $_POST[\'language\'] ) ) {
        update_post_meta( $order_id, \'language\', sanitize_text_field( $_POST[\'language\'] ) );
    }
}

然后。。。（这部分有点老套，但我从电子邮件创建的开始（主题）开始，然后将语言更改为订单的语言）。我不认为您真的需要担心更改回它，因为一旦线程死亡，它将切换回默认状态。它应该在发送完这封邮件后立即消失，因为这只会在回拨时发生。

add_filter(\'woocommerce_email_subject_customer_completed_order\', \'waspdigital_order_complete_language_fix\', 1, 2);
function waspdigital_order_complete_language_fix( $subject, $order ){
    global $woocommerce;
    $new_locale = get_post_meta( $order->id, \'language\', true);
    // set the current locale and send email with it active
    unload_textdomain(\'woocommerce\');
    setlocale(LC_ALL, $new_locale);
    global $q_config, $locale, $woocommerce;
    $locale = $new_locale;
    $q_config[\'language\'] = substr($new_locale, 0, 2);
    $woocommerce->load_plugin_textdomain();

/*
    // set the old locale back
    unload_textdomain(\'woocommerce\');
    $q_config[\'language\'] = substr($old_locale, 0, 2);
    $locale = $old_locale;
    setlocale(LC_ALL, $old_locale);
    $woocommerce->load_plugin_textdomain();
*/

    return $subject;
}