Invalid translation language (fa) specified
Basically, this error appears when some translation methods like $entity->getTranslation('LANGUAGE_CODE') or $entity->addTranslation('LANGUAGE_CODE') are called and there is no translation of the entity in the specified language.

The solution is to check if the entity hasTranslation using $entity->hasTranslation('fa') before calling the getTranslation or addTranslation like this:

if ($entity-&gt;hasTranslation('fa')) {
  $translation = $entity-&gt;getTranslation('fa');
}