Be suprised if spammers get past the human verification unless they are manually entering the spam. Lazarus was built with anti spam in mind and has two layers of invisible anti spam and human verification as an option you can set in the admin section.
You dont say which version of the guestbook you are using. If it's 2.2 I suggest you read the thread entitled read this before posting any advanced guestbook questions
Have a look at Lazarus. It comes with an include file for putting the guestbook in any page. This way you could easily just put it in a page for each language.
Doesn't help that you have put style sections everywhere. You only needed to add the body CSS to the already existing style block in header.php. Works fine once you remove all the excess.
You cannot make the background of a webpage transparent. You can make a tables background transparent to show the background image/colour of the body though.
Did you edit either of the mentioned files and if so what did you use? Things like cPanels filemanager have a nasty habit of putting extra lines onto the end of files when you save them.