Entity references should be displayed translated on non translatable entity types

1. +++ b/core/lib/Drupal/Core/Entity/Entity/EntityViewDisplay.php
   @@ -249,7 +250,13 @@ public function buildMultiple(array $entities) {
   +          // Display fields for untranslatable entities in the current content
   +          // language.
   

   Yeah, actually this is not the language for the field values (since the entity is not translatable, the values don't depend on language), but the language that the formatter should use for "what it does with the field values".

   This area is a bit confusing, and we've been adjusting it back and forth a couple times, so IMO this deserves some explicit comment here.

   Here's what I can come up with atm:
   "The language of the field values to display is already determined in the incoming $entity. The formatter should build its output of those values using:
   - the entity language if the entity is translatable,
   - the current "content language" otherwise."

   Suggestions / refinements welcome :-)

2. +++ b/core/lib/Drupal/Core/Entity/Entity/EntityViewDisplay.php
   @@ -249,7 +250,13 @@ public function buildMultiple(array $entities) {
   +          $view_langcode = $entity->language()->getId();
   +          if ($entity instanceof TranslatableInterface && !$entity->isTranslatable()) {
   +            $view_langcode = NULL;
   +          }
   

   Nitpick, but I'd find a ternary, or if too long an explicit if/else, to be clearer.

   Also, what if *not* instanceof TranslatableInterface ? we keep the entity langcode as if the entity was translatable ? seems weird ?
   Shouldn't it be :

   if ($entity instanceof Translatable && $entity->isTranslatable()) {
     $view_langcode = $entity->language()->getId(); 
   }
   else {
     $view_langcode = NULL;  
   }
   

   ?

Also, what if *not* instanceof TranslatableInterface ? we keep the entity langcode as if the entity was translatable ? seems weird ?

Aside from the fact that this should not be very common, I think it would probably make sense not provide a language also in this case. The entity language can be retrieved from the items if needed.

@plach : meaning, you're +1 on the proposal at the end of #7 ? (just making sure ;-)

Addressed the feedback from #7. Tests still needed.

Yep, that was a verbose +1 :)

Added tests.

I also encountered this problem while working on #2144377: Entity reference autocomplete lists entity labels only in current content language, and the fix looks good, as agreed upon in #9/#11.

Great to see this RTBC.

Tagging with rc target triage. It's not a huge problem, but we did run into both this and #2144377: Entity reference autocomplete lists entity labels only in current content language in our very first multilingual D8 project and I think many other sites will too if you e.g. use term references on users.

It's a small change in the behavior, but I don't think anyone expects the current behavior and we have agreement from @yched, @plach, @amateescu and @GaborHoitsy also agreed in IRC.

Discussed with the D8 committers and we agreed with making this an rc target.

Committed/pushed to 8.0.x, thanks!

Yay, thanks!