Experimental migrate_drupal_multilingual module

Problem/Motivation

Even after we get the last couple multilingual issues resolved, there are still many unknowns and multilingual areas could get improved. Rather than rushing and stating all multilingual migrations are ready because we just need to get things out the door, let’s take off the pressure. Give us a little more time to do things properly. Having a separate marking module let’s us mark multilingual stable when it is really more stable.

Proposed resolution

• Add a new migrate_drupal_multilingual module
• In all i18n and entity translation migrations add a tag 'Multilingual'
• In checkRequirements() check if the 'Multilingual' tag exists and if so add a requirement that the 'migrate_drupal_multilingual' module is enabled.
• If the module doesn’t exist (i.e. it isn’t installed) then it fails requirements and will not be available for migrations.
• Then mark migrate_drupal_multilingual as experimental. And mark migrate_drupal as stable.
• Profit

The original proposed resolution was limited to i18n migrations and for those, the i18n source plugins would have a requirement of the new module. It was then pointed out in #78 that entity translations also needed to be included. After a migrate meeting it was proposed to add a tag for entity translation, see #80. Then in both #81> and #86 it was questioned why two methods (testing the source plugin and using a tag) was necessary. Finally, in #87 the use of just the tag was agreed and implemented.

https://www.drupal.org/project/drupal/issues/2953360#comment-12652339 📌 Experimental migrate_drupal_multilingual module Fixed

Remaining tasks

• Roll a patch.
• Discuss if this can land as Experimental into 8.6. That way we could mark migrate drupal and the UI as stable sooner.

User interface changes

None.

API changes

None.

Data model changes

None.