"Language neutral" entities should not be translatable via the tmgmt_entity module

Fully agree with your description, this seems a severe bug.
Please consider: All bugs, especially such major ones, are required to provide test coverage.
:-)

Thanks, committed to 7.x-1.x.

Ported to 8.x, fixed the link of the source entity and the source entities with undefined source language are not displayed anymore.

Looks good.

There is one thing that I forgot, 8.x also has other special languages that we should exclude. Like LANGCODE_NOT_APPLICABLE. Instead of hardcoding it, I think we should compare it with the languageList() from the language manager, that's the same list as we're using in tmgmt_content_get_translatable_entities().

The at least the list test should then be extend to also include an entity with that language code.

Excluded the entities with not applicable languages and made the test.

+++ b/sources/content/src/Plugin/tmgmt/Source/ContentEntitySource.php
@@ -64,8 +64,8 @@ class ContentEntitySource extends SourcePluginBase {
-    if (isset($languages[$id])) {
-      throw new TMGMTException(t('Entity %entity could not be translated because its language is not applicable', array('%entity' => $entity->label())));
+    if (!isset($languages[$id])) {
+      throw new TMGMTException(t('Entity %entity could not be translated because the language ' . $entity->language()->getName() . ' is not applicable', array('%entity' => $entity->label())));
     }

You still need to update this to use a @language placeholder.

Changed the placeholders.

1. +++ b/sources/content/src/Plugin/tmgmt/Source/ContentEntitySource.php
   @@ -57,6 +59,14 @@ class ContentEntitySource extends SourcePluginBase {
   +    if ($entity->language()->getId() == LanguageInterface::LANGCODE_NOT_SPECIFIED) {
   +      throw new TMGMTException(t('Entity %entity could not be translated because it is language neutral', array('%entity' => $entity->language()->getId())));
   +    }
   +    $languages = \Drupal::languageManager()->getLanguages();
   +    $id = $entity->language()->getId();
   +    if (!isset($languages[$id])) {
   +      throw new TMGMTException(t('Entity %entity could not be translated because the language %language is not applicable', array('%entity' => $entity->language()->getId(), '%language' => $entity->language()->getName())));
   +    }
   

   You don't need both conditions here, the first one is a subset of the second and also covered with that (because not specified is not in the list of languages).

   Also, the %entity change in the first message was wrong.

2. +++ b/sources/content/tmgmt_content.module
   @@ -86,7 +86,7 @@ function tmgmt_content_get_translatable_entities($entity_type_id, $property_cond
    
   -  $langcodes = array_keys(language_list());
   +  $langcodes = array_keys(\Drupal::languageManager()->getLanguages());
      $languages = array_combine($langcodes, $langcodes);
    
   

   Avoid changing existing code that is not really related to an issue. This will actually conflict with #2427153: Replace deprecated calls, but this needs an update anyway.

Needs review...

Committed.