Hey there.
It's been a while since I last worked with this plugin, and even more time since I last reviewed this specific part of the code. However, by reading DKTools page on their plugin and taking a quick look at my code, I think you might need just a small change to get it working.
From what I gathered, DKTools use tags with a single pair of {} around it to replace the texts... right? Does it work with json files for each language set?
First, you need to make sure you have objects defined in each language file with the same structure as a single dedicated external json file for book data. This is important, as that is what the plugin will actually use to find the correct text to use. You are getting an empty text because the plugin didn't find any data for the given tag.
Second, set TAA_BookMenu 'Localization Escape Code' to {{key}} (with two pairs, and no '#' at the beginning as is its default). That because my plugin will replace the {key} with the object path for each component of the scenes (categories, titles, book texts, and so on). For example, it will process the tag and replace it with "{library.books.BookTitle1.title}" to get to the "Book Title 1" title text ({key} will be replaced by library.books.BookTitle1.title, so it will then run the tag {library.books.BookTitle1.title} through the localization plugin to find the correct language text for it).
Hope this is of any help!