My apologies. I was thinking the way to do this was exactly the same as it is in your PostBooks version. Instead, in your hosted demo database you will need to modify the “Default” Locale so it points to your language. You can find the “Default” Locale on this menu path: System > Setup > Locales.
Every hosted demo user is automatically assigned to the “Default” Locale. So, if you change the “Default” Locale in your hosted demo database so it matches the Locale you have set up on your PostBooks version, that should work. Try it and let me know if you are successful.