Gwolle-GB is the WordPress guestbook you've just been looking for. Beautiful and easy.
First, it's not a heavy load for the database, and second, it's that if you clicked wrong you're not lost without the possibility to 'undo' it via your database interface.
I thought about that problem and came up with the solution of Recaptcha. It helps you and your visitors help them. It's really THAT easy. Second, I integrated Akismet and it works like a charm. Fighting spam has never been easiser!
Although the plugin tries to automatically calculate the URL of your guestbook it may be that it doesn't get this job done well. For users who have that problem there's a new option integrated (since version 0.9.4.3) that allows you to set the complete URL of your guestbook manually.
A good start to learn about localization and WP plugins is the guide "Localizing a WordPress plugin using poEdit" (http://weblogtoolscollection.com/archives/2007/08/27/localizing-a-wordpress-plugin-using-poedit/) and also the WordPress documentation.