This is a problem if you don't choose a language before or your language file is not present.
So please be sure that;
- the file \xampp\security\htdocs\lang.tmp exists and contains your language code
- your language file relating to the containing language code exists in \xampp\security\htdocs\lang\
You can take a look into your PHP \xampp\php\logs\php_error_log
for information about what is missing too.
P-S.: I would not use the XAMPP security pages. They can produce some other problems and will probably not add any additional security at all if you use XAMPP the way it is meant for.