The Book Language pull-down menu controls the language of the book created by Gedcom Publisher. The pull-down menu includes choices based on the set of language files included in Language folder(s). The program folder includes a Language subfolder, and you may add a Language subfolder under your Gedcom Publisher folder. For information about customizing the languages used with Gedcom Publisher, see the Translations page.
Book Language, described above, sets the language used when creating the book. The Customize > Language menu command chooses the language for the Gedcom Publisher program itself.
Some genealogy programs support multiple "sentence languages", where facts/events/tags have sentence templates for multiple languages. When Gedcom Publisher reads a data file that has sentences defined in multiple languages, it will use the Sentence Language property to determine which set of sentence templates to use.