Determine language for error docs / standard index.html file by users language
If I got it right, ISPConfig is using the language which is defined under /usr/local/ispconfig/server/lib/config.inc.php to set up the error docs / index.html.
Instead, I think it would be better, if the provided language of the reseller could be used, when adding a new customer. And to use the standard language when he doesn't.