Support entity translation of paragraphs

I added this to our team board. Not yet scheduled for immediate progress. :-)

Note: When implementing this, pay special attention to the delete (confirm, restore) workflows.

The more we are thinking about this, the more challenging this task seems.

See also this nice task at field collection module with almost 300 replies and work in progress for 3 years...
#1344672: Field Collection: Field translation (entity_translation) support.

As stated overthere, an own field type might simplify a bit...
#2430209: Create own fieldtype for Paragraphs

But also these tasks show a bit what complexity we touch:
#2429335: Deletion of referenced entities
#2428833: Unable to 'save and keep published' after editing content with paragraphs in testing
And we have the extra revision topic... #2426631: Add revision support using entity reference revision module

Multilingual is for many of these cases another dimension added in complexity.

I guess we will just try it and quickly learn from failing. ;-)

Hint, paragraphs shows up on the list of translatable entity types. If i enable it though after save, the fields are not translateable and then when editing a second time, the checkbox is also reset.

Thoughts about our UX goal:
As a site builder i define some fields of my paragraph bundles as multilingual, with more than one language enabled. (BROKEN NOW)
As a translator, if i want to create a translation of a node with paragraphs, i
- have the node created, with language assigned
- click on translate tab from node detail
- click on Add for new translation language
- expect to have all paragraphs prefilled with original language

Now the problem is the composite relationship between the node and paragraph entities. As long as the reference relationship is not translatable, this relationship needs to be frozen when translating, meaning when translating, i
- can not remove/delete a paragraph
- can not add a paragraph
- can not switch type of a paragraph (upcoming feature)

Note that if the entity reference is translatable, the limitations above do not apply, but there are many other issues related with it. Thus we recommend to start with a clean implementation of the description above.

Created followups
#2461689: demo module: Multilingual setup
#2461695: Support asymmetric translations
#2463491: Hide add/remove buttons when translating
#2463575: Wrong "all languages" in multilingual paragraphs
All to clearly reduce the scope of this issue.

The README should explain how to properly configure multilingual paragraphs.
And outline the limitations with reference to #2461695: Support asymmetric translations

Hm, there's a fail in the test run.

And see my comment from #16, we need the readme extended.. Or a separate documentation task.

We should push this in ASAP to make paragraph translation work.

Some readme feedback. :-)

1. +++ b/README.txt
   @@ -9,3 +9,29 @@ When you use the Entity Reference Paragraphs widget + Entity Reference selection
   + * Enable the Paragraph module and TMGMT module.
   ...
   + * Check Paragraphs as the embedded reference in Configuration » Translation Management settings.
   

   Paragraphs work without TMGMT. This module should not have any reference in the configuration to TMGMT at all.

2. +++ b/README.txt
   @@ -9,3 +9,29 @@ When you use the Entity Reference Paragraphs widget + Entity Reference selection
   + * Enable Paragraphed article to be translatable in Configuration » Content language and translation:
   

   Paragraphed article is only an example content type from the demo module.

   Either you refer to the demo setup... Or you refer to any custom content type with a paragraph field.

3. +++ b/README.txt
   @@ -9,3 +9,29 @@ When you use the Entity Reference Paragraphs widget + Entity Reference selection
   +	- Under Content check Paragraphed article
   

   Tab here...

4. +++ b/README.txt
   @@ -9,3 +9,29 @@ When you use the Entity Reference Paragraphs widget + Entity Reference selection
   +   - The fields inside the Paragraph block should be set to translatable
   

   Better: Set the fields of each paragraph type to translatable as required.

5. +++ b/README.txt
   @@ -9,3 +9,29 @@ When you use the Entity Reference Paragraphs widget + Entity Reference selection
   +For now, this module does not support switching entity reference revision field of the paragraph itself into multilingual mode. This would raise complexity significately.
   

   Wrong line break.

Aww... Providing interdiff.
The paragraphs_demo module is first used in this test. It had implicit dependency to path and menu_ui module.. And paragraphs_demo shipped config through views config, but still with only default values. Dropped it.

Still depends to the core patch. :-)

OK, nice, ready to commit. formally marking it postponed, adding related.

Aww... Strange side effects here...

• when adding a translation, also the origin language is overwritten.
• when editing a translation, also my wysiwyg fields get empty, although the previous value was stored and shown on display.
• then editing again, the translation is empty. But when filled out again and saved, things work again regularly.

Let's start with adding a test that checks the first two buggy steps.

This is needed to make the failing tests pass.

Prfct!

Unpostponing this:
- commenting the UI checking lines. It still works.
- create followup to reenable: #2473721: Check multilingual settings for paragraphs

Yay! Party! Welcome to the multilingual world! Thx for pushing! Me Xited! ;-)