I'm trying to add "fa" language in a Drupal 8 installation. I added it from admin/config/regional/language. Everything is OK and the language added successfully and added in language switcher in front-end. But when I click on it, I get this error:
InvalidArgumentException: Invalid translation language (fa) specified. in Drupal\Core\Entity\ContentEntityBase->getTranslation() (line 748 of core/lib/Drupal/Core/Entity/ContentEntityBase.php).
Note: This is not a clean installation. The Drupal I have is a working site with some modules which I want to add Persian as its 6th language. Any suggestions?