Create separate sitemaps in WordPress for all the activated languages in WPML.

wordpress_sitemaps.php

<?php
function sitemaps() {
	/* get all the activated languages in WPML plugin */
	$languages = apply_filters( 'wpml_active_languages', NULL, array( 'skip_missing' => 0));

	foreach((array)$languages as $lang) {

	    /* change language */
	    do_action('wpml_switch_language', $lang['code']);
	    
	    /* building query */
	    $posts = new WP_Query(array(
	        'sort_order' => 'ASC',
	        'sort_column' => 'post_title',
	        'post_type' => ['post', 'page'],
	        'posts_per_page' => -1,
	        'post_status' => 'publish',
	    ));
	    $posts = $posts->posts;

	    /* create dom document for XML */
		$dom = new DOMDocument();
		$dom->encoding = 'utf-8';
		$dom->xmlVersion = '1.0';
		$dom->formatOutput = true;

		/* sitemap.xml for English version */
		if($lang['code'] == 'en')
			$xml_file_name = "sitemap.xml";
		else
			$xml_file_name = "sitemap_".$lang['code'].".xml";
		
		$urlset = $dom->createElement('urlset');
		
		$attr_1 = new DOMAttr('xmlns:xsi', "http://www.w3.org/2001/XMLSchema-instance");
		$urlset->setAttributeNode($attr_1);

		$attr_2 = new DOMAttr('xmlns:image', "http://www.google.com/schemas/sitemap-image/1.1");
		$urlset->setAttributeNode($attr_2);

		$attr_3 = new DOMAttr('xsi:schemaLocation', "http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd http://www.google.com/schemas/sitemap-image/1.1 http://www.google.com/schemas/sitemap-image/1.1/sitemap-image.xsd");
		$urlset->setAttributeNode($attr_3);

		$attr_4 = new DOMAttr('xmlns', "http://www.sitemaps.org/schemas/sitemap/0.9");
		$urlset->setAttributeNode($attr_4);

	    foreach((array)$posts as $post) {
	    	
			$date = str_replace(' ', 'T', $post->post_modified_gmt);
	    	$date = $date."+00:00"; //according to Google Sitemap requirement
	    	$image = get_the_post_thumbnail_url($post->ID);
	    	$url = esc_url(get_page_link($post->ID));

			$url_node = $dom->createElement('url');
			
			$child_node_url = $dom->createElement('loc', $url);
			$url_node->appendChild($child_node_url);

			$child_node_date = $dom->createElement('lastmod', $date);
			$url_node->appendChild($child_node_date);

			$urlset->appendChild($url_node);
			
			if($image != '') {
				$image_node = $dom->createElement('image:image');
				$url_node->appendChild($image_node);

				$child_node_image = $dom->createElement('image:loc', $image);
				$image_node->appendChild($child_node_image);
			}
	    }
		$dom->appendChild($urlset);
		$dom->save($xml_file_name);
	}
}