InvalidArgumentException: Invalid translation language (und) specified.

Same here, using search_api-8.x-1.0-beta2 with database source.

Same here:

InvalidArgumentException: Invalid translation language (en) specified. in Drupal\Core\Entity\ContentEntityBase->getTranslation() (line 745 [error]
of /core/lib/Drupal/Core/Entity/ContentEntityBase.php).
An error occurred during indexing: Invalid translation language (en) specified.

Using 8.x-1.0-beta2 with database backend

Here is a backtrace

/docroot/modules/contrib/search_api/src/Utility/FieldsHelper.php(101): Drupal\Core\Entity\ContentEntityBase->getTranslation('en')
#1 /docroot/modules/contrib/search_api/src/Utility/FieldsHelper.php(143): Drupal\search_api\Utility\FieldsHelper->extractFields(Object(Drupal\Core\Entity\Plugin\DataType\EntityAdapter), Array, 'en')
#2 /docroot/modules/contrib/search_api/src/Utility/FieldsHelper.php(148): Drupal\search_api\Utility\FieldsHelper->extractFields(Object(Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem), Array, 'en')
#3 /docroot/modules/contrib/search_api/src/Utility/Utility.php(93): Drupal\search_api\Utility\FieldsHelper->extractFields(Object(Drupal\Core\Entity\Plugin\DataType\EntityAdapter), Array, 'en')
#4 /docroot/modules/contrib/search_api/src/Item/Item.php(239): Drupal\search_api\Utility\Utility::extractFields(Object(Drupal\Core\Entity\Plugin\DataType\EntityAdapter), Array, 'en')
#5 /docroot/modules/contrib/search_api/src/Plugin/search_api/processor/RenderedItem.php(273): Drupal\search_api\Item\Item->getFields()
#6 /docroot/modules/contrib/search_api/src/Item/Item.php(245): Drupal\search_api\Plugin\search_api\processor\RenderedItem->addFieldValues(Object(Drupal\search_api\Item\Item))
#7 /docroot/modules/contrib/search_api/src/Entity/Index.php(951): Drupal\search_api\Item\Item->getFields()
#8 /docroot/modules/contrib/search_api/src/Entity/Index.php(911): Drupal\search_api\Entity\Index->indexSpecificItems(Array)
#9 /docroot/modules/contrib/search_api/src/IndexBatchHelper.php(155): Drupal\search_api\Entity\Index->indexItems(1)
#10 /vendor/drush/drush/commands/core/drupal/batch.inc(156): Drupal\search_api\IndexBatchHelper::process(Object(Drupal\search_api\Entity\Index), 1, 2036, Object(DrushBatchContext))
#11 /vendor/drush/drush/commands/core/drupal/batch.inc(104): _drush_batch_worker()
#12 /vendor/drush/drush/includes/batch.inc(92): _drush_batch_command('468')
#13 /vendor/drush/drush/commands/core/core.drush.inc(1207): drush_batch_command('468')
#14 /vendor/drush/drush/includes/command.inc(366): drush_core_batch_process('468')
#15 /vendor/drush/drush/includes/command.inc(217): _drush_invoke_hooks(Array, Array)
#16 /vendor/drush/drush/includes/command.inc(185): drush_command('468')
#17 /vendor/drush/drush/lib/Drush/Boot/BaseBoot.php(67): drush_dispatch(Array)
#18 /vendor/drush/drush/includes/preflight.inc(66): Drush\Boot\BaseBoot->bootstrap_and_dispatch()
#19 /vendor/drush/drush/drush.php(12): drush_main()
#20 {main}InvalidArgumentException: Invalid translation language (en) specified. in Drupal\Core\Entity\ContentEntityBase->getTranslation() (line 747 of                                              [error]
/docroot/core/lib/Drupal/Core/Entity/ContentEntityBase.php).

I attach a quick patch that before trying to get a field translation query if it exits, this avoid the error, but i dont know what to do when there is not translation.

I think we can close this as a duplicate of #2684465: Taxonomy terms are not indexed as expected for multilingual sites. ?

Confirmed in irc by @Dropa to be a duplicate. Closing this one.