Migrate D6 languages

I will give a try.

Languages are really special config entities. We have Language classes, and entity classes. Because of circular dependencies between language system and configuration, entities are only used for loading/saving into config system, but in runtime we use the Drupal\Core\Language\Language objects.

For this reason, we need to clear language_list cache. ATM I put that on the migration test, but ideally this should happen after import (I don't know if we have mechanisms for running something after migration execution). We probably need to do other things as well (update state()->language_count?

Also, this is incomplete because in D6 we had language negotiation information in the languages table, and now it is outside of the language entities.
#1862202: Objectify the language system is a big refactoring around language negotiation really close to land, so maybe we should open a follow-up for that and postpone it on that if we want to move this forward.

Tests for the language source plugin are still missing.

Included language source test.

In the case of languages, we shouldn't use the machine_name plugin but just copy language to id.
If we install in English (en) and we execute a migration, we don't want a new language English(en1), but to override English(en) configuration.

3: imp-2166875-languages-3.patch queued for re-testing.

Change explained at #4: don't use machine_name but get the language id as is.

With #7 language migration should work.
However, language config and language negotiation configuration is not taking place yet.

I would say that language config should happen in a different migration, maybe triggered by this one (or the opposite) for consistence.

About language negotiation settings, we should do that in another issue, not sure if it will affect this migration or will be a completely new one. Anyway that would be postponed on #1862202: Objectify the language system.

Still blocked on #2108599: Convert language_default to CMI and #2102477: Convert remainder of language negotiation settings to configuration system.

I guess this is doable now.

Updated the language migration after the changes in the last weeks.
The current patch still is red.

The problem was with the clearing of language caches.
I added a core issue #2234623: Move language add logic out of language_save() and marked here with a ToDo.

Added a patch in the core issue, which is not green yet but makes this one pass without clearing language caches.

New patch. The only difference is moving the migration to config/install, no interdiff attached.

Comment #18... not sure what we should do. I guess we can document that config migrations should be run on install? Should we create English always ìnstead as it was in D6 and D7?

Comment #19 should be take care of in #2225293: Migrate D6 and D7 language negotiation settings.

Overriden runQuery() for adding English if not present at the sources time and using ArrayIterator instead.

Changed test languages for avoiding English there. Anyway, the current tests don't test this behavior, as the tests installation is always done in English so that's why English is present. Needs manual testing.

This will fail per #2272219: Use iterator_count() instead of count() in MigrateTestCase::queryResultTest, still investigating.

Now it is green, it needed to override count() in Drupal\migrate_drupal\Plugin\migrate\source\d6\Language.