Change language of site with a html button

you can do this by

<a href="index.php?language=en">
<a href="index.php?language=no">

and get the languages and store them in cookie and include file according to cookie like

if ( !empty($_GET['language']) ) {
    $_COOKIE['language'] = $_GET['language'] === 'en' ? 'en' : 'nl';
} else {
    $_COOKIE['language'] = 'nl';
}
setcookie('language', $_COOKIE['language']);

and than

if ( $_COOKIE['language'] == "en") {
   include("headerEn.php");
} else {
   include("header.php");
} ?>

To give a solution without changing your approach, You can do like this.

<?php 
if(isset($_GET['language']))
  $language = $_GET['language'];
else
  $language = "";

if ($language == "en") {
   include("headerEn.php");
} else {
   include("header.php");
} ?>

<a href="index.php?language = en"><img src="images/language/languageNO.png">      </a>
<a href="index.php?language = no"><img src="images/language/languageEN.png"></a>

If you want to keep the selection, you can store this value in Database or Session.

It is always good to have a default value, so that you never end up being in a no-language site.

$language = $_REQUEST["language"];
$default_header="myheaderXXX.php";

switch ($language) {
    case "en":
      include("headerEn.php");
      break;

    case "no":
      include("header.php");
      break;

    default:
      include($default_header);
}

And then create links like this:

<a href="index.php?language=en">
<a href="index.php?language=no">