How can i add a cookie in this code?

Hey All,

I have been trying to send a cookie when somebody selects a "new" language from the language switcher.
I do this because i have htaccess RewriteCond based on that cookie

ok i have something like this:
setcookie('i18n_lang', '#value', time() + 604800);

But i'm really bad in coding and tried it everywhere... but none seem to work correct. Where could i place this code?