for better understanding you can replace the include with the content of the included file, so
//language.php
$message = 'hello :' . $name;
//require.php
include('language.php');
$name = 'booyah';
mail ($to, $sub, $message, 'From: [email]me@me.com[/email]');
becomes
$message = 'hello :' . $name;
$name = 'booyah';
mail ($to, $sub, $message, 'From: [email]me@me.com[/email]');
got it now?