that's right, u'll need register {user_realname} in "main" .php files.
Just search for: $site_template->register_vars(array(
then u'll see bunch of "something" => $whatever,
"sometingelse" => $somethingelse,
just add this line there:
"user_realname" => $lang['user_realname'],
make sure, that if u entered as last line in the array, there is no comma at the end of your line, and line above yours must have comma.