Currently the intended pattern is for book texts to be provided in the book provider.
The book provider takes one lang currently, that should be improved.

I think it should offer a map of lang code -> lang provider that can be accessed in the book provider.

BookProvider now first takes a default language provider accessible via .lang and .lang(), and additional translation language providers that are accessible via their locale

        this.lang().add(helper.bookName(), "Demo Book");
        this.lang().add(helper.bookTooltip(), "A book to showcase & test Modonomicon features.");

        this.lang("ru_ru").add(helper.bookName(), "Демонстрационная книга");
        this.lang("ru_ru").add(helper.bookTooltip(), "Книга для демонстрации и тестирования функций \"Модономикона\".");