Layout definition defined in YAML is not translated

I think it should be broader issue to allow define translatable properties in annotations

\Drupal\Core\Layout\Annotation\Layout defines label & description as @ingroup plugin_translatable but actually it makes it translatable as https://github.com/Atlantic18/DoctrineExtensions/blob/master/doc/transla... supposed

Let's see how many tests will fail

Also region names needs wrapper somehow

A bit cleaner patch

Fix test

Looking deeper it looks like \Drupal\Core\Plugin\Discovery\YamlDiscoveryDecorator should be able to detect properties that are translatable
Also reflection of properties annotations could bring overhead of yaml discovery

First of all, this needs tests.

Second, this should (ideally) work.

There's no BC break according https://www.drupal.org/core/d8-bc-policy#public-properties because \Drupal\Core\Layout\LayoutDefinition has no interface

Fix #10 s/title/label and extended tests

Thanks @andypost!

I believe breakpoint.module suffers from the same oversight, another issue should be opened for that module and any others using \Drupal\Core\Plugin\Discovery\YamlDiscovery

Filed follow-up #3009291: Fix remaining untranslatable properties from YAML discovery

At least region names should have own translation context

I did not add "context_key" argument to addTranslatableProperty() because it used in a strange way in \Drupal\Core\Plugin\Discovery\YamlDiscovery::getDefinitions()

            $options = [];
            // Move the t() context from the definition to the translation
            // wrapper.
            if ($context_key && isset($definition[$context_key])) {
              $options['context'] = $definition[$context_key];
              unset($definition[$context_key]);
            }
            $definition[$property] = new TranslatableMarkup($definition[$property], [], $options);

And fixed test

This is an accessibility must-have. We'll need to use layout/region label/description to describe various aspects of the layout builder UI to screen reader users.

Not sure what this has to do with accessibility, but I'll take your word for it :)

Committed 1b186cf and pushed to 8.7.x. Thanks!

C/p as 090bbaea43 and pushed to 8.6.x