A config mapper can only contain configuration for a single language

Problem/Motivation

Step to reproduce:
1- Install a drupal 8 in Spanish.
2- In "admin/config/regional/language/add" add Inglés language.
3- Go to "/en/admin/modules" install "Configuration Translation" and "Content Translation" modules.
4- Go to "/en/admin/structure/types/manage/page/fields", click in "Add field", in "Re-use an existing field', select "Image: Field image", "Save and continue".
5- Bump!

RuntimeException: A config mapper can only contain configuration for a single language. in Drupal\config_translation\ConfigNamesMapper->getLangcode() (line 398 of core/modules/config_translation/src/ConfigNamesMapper.php).

Drupal\config_translation\Access\ConfigTranslationOverviewAccess->access(Object, Object)
call_user_func_array(Array, Array)
Drupal\Core\Access\AccessManager->performCheck('config_translation.access.overview', Object)
Drupal\Core\Access\AccessManager->check(Object, Object, NULL, 1)
Drupal\Core\Access\AccessManager->checkNamedRoute('entity.field_config.config_translation_overview.node', Array, Object, 1)
Drupal\Core\Menu\LocalTaskManager->getTasksBuild('entity.field_config.node_field_edit_form', Object)
Drupal\Core\Menu\LocalTaskManager->getLocalTasks('entity.field_config.node_field_edit_form', 0)
Drupal\Core\Menu\Plugin\Block\LocalTasksBlock->build()
Drupal\block\BlockViewBuilder::preRender(Array)
call_user_func('Drupal\block\BlockViewBuilder::preRender', Array)
Drupal\Core\Render\Renderer->doRender(Array)
Drupal\Core\Render\Renderer->doRender(Array, )
Drupal\Core\Render\Renderer->render(Array)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1)
__TwigTemplate_0f04497e0607dbb55ae785558deed68eacde76e3989f8c8e0779ccf1b6af549c->doDisplay(Array, Array)
Twig_Template->displayWithErrorHandling(Array, Array)
Twig_Template->display(Array)
Twig_Template->render(Array)
twig_render_template('core/themes/seven/templates/page.html.twig', Array)
Drupal\Core\Theme\ThemeManager->render('page', Array)
Drupal\Core\Render\Renderer->doRender(Array, )
Drupal\Core\Render\Renderer->render(Array)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1)
__TwigTemplate_16436f2456eb6e9f1287d658a3b71677eb9755e9eb4796bd9e27a1209ebeaf7f->doDisplay(Array, Array)
Twig_Template->displayWithErrorHandling(Array, Array)
Twig_Template->display(Array)
Twig_Template->render(Array)
twig_render_template('core/themes/classy/templates/layout/html.html.twig', Array)
Drupal\Core\Theme\ThemeManager->render('html', Array)
Drupal\Core\Render\Renderer->doRender(Array, )
Drupal\Core\Render\Renderer->render(Array)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}()
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.view', Object)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)

Fields have two config objects,
the "Edit" tab is field.field... and
the "Field settings" the field.storage...

When installing in spanish, using standard profile, the field and storage config have langcode es.
After adding english, and enabling configuration translation module, and specifying en in the admin url, the field config (not storage) has the langcode en.
The translate tab, then errors because it expects all the config on the form to have the same source.

This is only a problem when config translation is enabled. (The field stuff is fine.)

Proposed resolution

Possible solution,
instead of a fatal error, give a warning. Wording the warning might be tricky. Maybe "Warning, the field configuration and field settings configuration have different source languages, and will not be able to be translated."

Remaining tasks

User interface changes

Adding a warning.

API changes

??

Data model changes

No.