Can you try:
<?php
parse_str($_SERVER['QUERY_STRING'], $vars);
$query = http_build_query($vars);
if ($query) $query = "?" . $query;
?>
<ul>
<li class="active"><a href="/en/<?php echo basename($_SERVER['PHP_SELF']).$query ?>">ENGLISH</a></li>
<li><a href="/de/<?php echo basename($_SERVER['PHP_SELF']).$query ?>">GERMAN</a></li>
<li><a href="/fr/<?php echo basename($_SERVER['PHP_SELF']).$query ?>">FRENCH</a></li>
</ul>