Codeigniter uses a fairly standard way of inserting values into strings within its language files.
You define your string with placeholders, and then use
sprintf to format the string.
//In your language file... $lang['welcome_username'] = 'Welcome %s.'; //then in your view or controller..
echo ( sprintf( lang("welcome_username"), "Bob") ); //Results in "Welcome Bob."
However, for values that only really change per environment / deployment, it would be beneficial to be able to refer to them from within the language file, rather than having to format the string every time you want to use it.