WordPress mu-plugin to change the WPML language code in the URL from ja to jp

akos-resch-wpml-subdir-ja-to-jp.php

<?php

/**
 * Change the Japanese subdirectory language code from ja to jp
 *
 * @param array $language_codes_map
 *
 * @return array
 */
function akos_resch_wpml_subdirectory_language_code_ja_to_jp( $language_codes_map ) {
	$language_codes_map['ja'] = 'jp';

	return $language_codes_map;
}

add_filter( 'wpml_language_codes_map', 'akos_resch_wpml_subdirectory_language_code_ja_to_jp', 10, 1 );