Since you want to Get a Book with a specific BookTranslation, so its better that you base your Query on BookTranslation and get the Book object from it.
i.e. Use something like this
class BookDetail(DetailView):
model = Book
def get_object(self):
self.booktranslation = BookTranslation.objects.select_related('book').get(book__slug=self.kwargs['slug'],
language=self.kwargs['language'])
return self.booktranslation.book
You can then pass self.booktranslation as a context variable to access that in template.