change_language_code_in_url

<?php

/**
 * Change the  subdirectory language code from es to mx
 *
 * @param array $language_codes_map
 *
 * @return array
 */
function argo_mx_lang( $language_codes_map ) {
	$language_codes_map['es'] = 'mx';

	return $language_codes_map;
}

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