I'm back with good news!
Since I moved my website online the guestbook url is a SEF one and I've solved the captcha display problem on EN language by setting the "Relative path" to "Relative patch (no language prefix)" directly from PhocaGuestbook settings.
But there's a problem with phocaguestbook extensions for joomsef. My website host has display errors turned on so that I could see this error which looks like this:
Warning: array_merge() [function.array-merge]: Argument #2 is not an array in /home/decora11/public_html/components/com_sef/sef_ext/com_phocaguestbook.php on line 113
The 113 line is the following:
$title=array_merge($title,$this->_processPagination($uri));
I've commented it and now the error is gone. But it's ok commenting this line?