WPML Support

I’ve written a little script to sync the stock between the languages, maybe you could adapt something like that

simple products:

function abBulkStockSimpleProducts($id) {
	global $sitepress;
	$languageIDs = array();
	$active_languages = apply_filters( 'wpml_active_languages', false );
	foreach($active_languages as $code => $data) :
		if($code != 'de') :
			$languageIDs[] = icl_object_id($id,'product',false,$code);
		endif;	
	endforeach;
	$new_quantity = get_post_meta($id, '_stock', true);
	
	if (is_numeric($new_quantity)) :
		$new_stock_status = ($new_quantity > 0) ? "instock" : "outofstock";
		wc_update_product_stock_status($id, $new_stock_status);
		if(count($languageIDs) > 0) :
			foreach($languageIDs as $langProd) :
				update_post_meta($langProd, '_stock', $new_quantity);
				wc_update_product_stock_status($langProd, $new_stock_status);
			endforeach;
		endif;
	endif;
	
}

Variation Products:

function abBulkStockAfterProcessQtyActionVariables($id){
	global $sitepress;
	if(ICL_LANGUAGE_CODE != 'de') :
		$germanID = get_post_meta($id, '_wcml_duplicate_of_variation', true);
	else :
		$germanID = $id;
	endif;
	$new_quantity = get_post_meta($germanID, '_stock', true);
	if (is_numeric($new_quantity)) :
		$new_stock_status = ($new_quantity > 0) ? "instock" : "outofstock";
		wc_update_product_stock_status($germanID, $new_stock_status);
		/*  Use the TRID to find the translated Product ids  */
		$trid = $sitepress->get_element_trid($germanID, 'post_product');
		if (is_numeric($trid)) :
			$translations = $sitepress->get_element_translations($trid, 'post_product');
			if (is_array($translations)) :
				/* Loop through each existing translation */
				foreach($translations as $translation) :
					if (!isset($translation->element_id) || $translation->element_id == $germanID) :
						continue;
					endif;

					/*  set the stock status and quantity in the translated by updating the post-meta info */
					update_post_meta($translation->element_id, '_stock', $new_quantity);
					wc_update_product_stock_status($translation->element_id, $new_stock_status);
				endforeach;
			endif;
		endif;
	endif;

}