我想出了一个函数来完成这项工作：

/**
 * Creates a translation of a post (to be used with WPML)
 *  
 * @param int $post_id The ID of the post to be translated.
 * @param string $post_type The post type of the post to be transaled (ie. \'post\', \'page\', \'custom type\', etc.).
 * @param string $lang The language of the translated post (ie \'fr\', \'de\', etc.).
 *    
 * @return the translated post ID
 *  */
function mwm_wpml_translate_post( $post_id, $post_type, $lang ){

    // Include WPML API
    include_once( WP_PLUGIN_DIR . \'/sitepress-multilingual-cms/inc/wpml-api.php\' );

    // Define title of translated post
    $post_translated_title = get_post( $post_id )->post_title . \' (\' . $lang . \')\';

    // Insert translated post
    $post_translated_id = wp_insert_post( array( \'post_title\' => $post_translated_title, \'post_type\' => $post_type ) );

    // Get trid of original post
    $trid = wpml_get_content_trid( \'post_\' . $post_type, $post_id );

    // Get default language
    $default_lang = wpml_get_default_language();

    // Associate original post and translated post
    global $wpdb;
    $wpdb->update( 
        $wpdb->prefix.\'icl_translations\', 
        array( 
            \'trid\' => $trid, 
            \'language_code\' => $lang, 
            \'source_language_code\' => $default_lang 
        ), 
        array( 
            \'element_type\' => $post_type, 
            \'element_id\' => $post_translated_id 
        ) 
    );

    // Return translated post ID
    return $post_translated_id;

}