Handle "A config mapper can only contain configuration for a single language" more gracefully

Problem/Motivation

After adding config entities (node types) to the cart, I get the following error at /admin/tmgmt/cart:

The website encountered an unexpected error. Please try again later.

Drupal\config_translation\Exception\ConfigMapperLanguageException: A config mapper can only contain configuration for a single language. in Drupal\config_translation\ConfigNamesMapper->getLangcode() (line 404 of core/modules/config_translation/src/ConfigNamesMapper.php).

Drupal\tmgmt_config\Plugin\tmgmt\Source\ConfigSource->getSourceLangCode() (Line: 994)
Drupal\tmgmt\Entity\JobItem->getSourceLangCode() (Line: 35)
Drupal\tmgmt\Form\CartForm->buildForm()
call_user_func_array() (Line: 534)
Drupal\Core\Form\FormBuilder->retrieveForm() (Line: 281)
Drupal\Core\Form\FormBuilder->buildForm() (Line: 73)
Drupal\Core\Controller\FormController->getContentResult()
call_user_func_array() (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 580)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext() (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 169)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 81)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 58)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass() (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 50)
Drupal\ban\BanMiddleware->handle() (Line: 270)
Drupal\shield\ShieldMiddleware->bypass() (Line: 169)
Drupal\shield\ShieldMiddleware->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 49)
Drupal\remove_http_headers\StackMiddleware\RemoveHttpHeadersMiddleware->handle() (Line: 23)
Stack\StackedHttpKernel->handle() (Line: 707)
Drupal\Core\DrupalKernel->handle() (Line: 19)

1. I guess the reason isn't in TMGMT but in the site config, that was upgraded from Drupal 7. But it would be nice to show a formatted error message above the cart or anything like that, instead of that hard exception.
Or is there any core issue / solution I don't know about?

2. Furthermore, it would be super helpful to see which translation / config / entity causes the Exception.

3. With that Exception, there's no way to clear the cart.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes