1. if ($lang == en) and if($lang == it)
if they're strings, they should be encapsulated in either single or double quotes.
any alphanums outside of quotes (and not prefixed by $) are tested to be functions, classes, keywords, and constants. if all of those fail (and i'm probably forgetting a few), PHP assumes they are meant to be strings, throws that E_WARN level error, and moves on.
Encapsulate your strings and you'll save yourself and the PHP engine the headache
2: $lang = 'en';
this line isnt inside <?php ?>, and wont be interpreted by the PHP engine.
3: if...if on exclusive condition.
if the lang is en, it wont be it. make your second if an elseif. It'll save your script having to process the second if condition.