wpml_set_element_language_details

add_action('wp_footer', 'element_connect_on_insert');

function my_insert_posts() {
	$output = array();

	// Create original post object
	$my_original_post = array(
		'post_title'    => 'My original post',
		'post_content'  => 'This is my original post.',
		'post_status'   => 'publish',
		'post_author'   => 1,
		'post_category' => array(1)
	);

	// Create translation post object
	$my_translated_post = array(
		'post_title'    => 'My translated post',
		'post_content'  => 'This is my translated post.',
		'post_status'   => 'publish',
		'post_author'   => 1,
		'post_category' => array(2) // NOTE: this is the translated category id!
	);

	// Insert the 2 posts into the database
	$original_post_id = wp_insert_post( $my_original_post );
	$translated_post_id = wp_insert_post( $my_translated_post );

	return $output = array(
		'original' => $original_post_id,
		'translation' => $translated_post_id
	);
}


function element_connect_on_insert() {
	$inserted_post_ids = my_insert_posts();

	if ( $inserted_post_ids) {
		// https://wpml.org/wpml-hook/wpml_element_type/
		$wpml_element_type = apply_filters( 'wpml_element_type', 'post' );
		
		// get the language info of the original post
		// https://wpml.org/wpml-hook/wpml_element_language_details/
		$get_language_args = array('element_id' => $inserted_post_ids['original'], 'element_type' => 'post' );
		$original_post_language_info = apply_filters( 'wpml_element_language_details', null, $get_language_args );
		
		$set_language_args = array(
			'element_id'    => $inserted_post_ids['translation'],
			'element_type'  => $wpml_element_type,
			'trid'   => $original_post_language_info->trid,
			'language_code'   => 'de',
			'source_language_code' => $original_post_language_info->language_code
		);

		do_action( 'wpml_set_element_language_details', $set_language_args );
	}
}