help me to create language switching button/link

I’m building multi-language blog and I still need the activate button(or)link which can let the users switch any language.

I’ve coded as below and when I click(or)switch my-language it’s still find, but when I reload to the new page my-language is disappear and it automatically changes to the default language.
can someone guide me to the right way, I think sth wrong with my code.

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// * header.php  * //
<a href="<?php echo $_SERVER["HTTP_HOST"] . $_SERVER["REQUEST_URI"]; ?>/?lang=mylanguage">French</a>
<a href="<?php echo $_SERVER["HTTP_HOST"] . $_SERVER["REQUEST_URI"]; ?>/?lang=eng">English</a>

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// * function.php  *//
<?php
session_start();

$_SESSION['language'] = $_GET['lang'];

if ( $_SESSION['language'] == 'mylanguage' ) {

require (TEMPLATEPATH .'/languages/my-language.php');

}
else {

require (TEMPLATEPATH .'/languages/english.php');  // default language

}
?>

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