Yes, it should be.
This plugin doesn’t use a custom post type, but it’s own database table for content, from a time before custom post types existed.
The FAQ at wp.org explains it well (I think):
I use a Multi-Lingual plugin
There are 2 settings that you need to pay attention to. If you saved the settings for the form tab, you should save an
empty header and notice text. It will fill in the default there after saving, but that is okay.
As long as you saved an empty option, or it is still not-saved, then it will show the translated text from your MO file.
Also, you will want to use the book_id parameter of the shortcode for multiple guestbook.