Not working Lang Changer in a Class Function PHP

Hello i try to implent a Language Changer in my Script.
But in my Class Function it is saying:
Notice: Undefined variable: lang in ......

I use a if else at begin of my Index.php which decide to includes the correct lang_xxx.php.

And in my lang_xx.php there is Array

    /* 
    -----------------
    Language: English
    -----------------
    */

    $lang = array();


$lang['welcome_msg'] = 'Welcome Guest';

And echo in my Class then

<?php echo $lang['welcome_msg']; ?>

So my Question why isnt this working and what is the correct way to implent this?

Its also not working when i include the lang_xx.php File directly in the Class Function.

via Chebli Mohamed