function change_language() {
	// find the name of the currently open page
    var actual_page = parent.main.location.href;
	var find_page_name_regex_string = "([A-Za-z0-9_]*\.(html|htm|php))";
  	var find_page_regex = new RegExp( find_page_name_regex_string );
    var page_results = find_page_regex.exec( actual_page );
	var page_name = page_results[0];
	
	// check language version, to know which document to choose
	var check_language_regex_string = "^(a_)";
	var check_language_regex = new RegExp( check_language_regex_string );
	var language = check_language_regex.exec ( page_name );
	
	// check last update date
	var update_date = document.getElementById("update_date");
	var check_date_regex_string = "[0-9]{2}\.[0-9]{2}\.[0-9]{4}";
	var check_date_regex = new RegExp( check_date_regex_string );
	var date = check_date_regex.exec ( update_date.innerHTML );
	
	var top_page;
	var menu_page;
	var flag = document.getElementById("flag");
	
	// polish version (document name without leading "a_"
	if (language == null) {
	     page_name = "a_" + page_name;
		 menu_page = "a_menu.html";
		 flag.src = "images/pl.gif";
		 flag.alt = "Polish Version";
		 update_date.innerHTML = "Last Update: <span style=\"font-weight: bold\">" + date + "</span>";
	}
	else {
		 page_name = page_name.substring(2);
		 menu_page = "menu.html";
		 flag.src = "images/eng.gif";
		 flag.alt = "English Version";
		 update_date.innerHTML = "Aktualizacja: <span style=\"font-weight: bold\">" + date + "</span>";
	}

	parent.main.location = page_name;
	parent.menu.location = menu_page;
}