Order matters ;P You can order the enabled languages in these system preferences.. the OPAC one is particular important as the order alters both the picker options order, the tab display order for translated notices and most importantly the first option in the list is the 'default' language of the system and is applied to the includes in default notices.
Page to add this information on: https://koha-community.org/manual/latest/en/html/i18npreferences.html