Migrate legacy languages to configurable language entites

As you can see in the related issue, we agreed on adding English even when no English was visible, as it is not safe to assume that there is no content in English.

Still needs #5.

Adding to sprint, as it is actively worked.

Today the D8 core committers agreed that patches that only change "Experimental" functionality (like Migrate) can be treated as "rc eligible" rather than rc targets. Reference: https://www.drupal.org/core/d8-allowed-changes#rc

With the patch in #6, all languages get migrated, also the disabled ones.

I did some tests with a D6 site with Dutch and English nodes, even though the English language was disabled.
All languages get migrated to Drupal 8, and all nodes get the proper language assigned to them.

So I don't see why we'd need to extend the source plugin to explicitly add the English language.
Virtually any Drupal 6 and Drupal 7 site will have English in the languages table (be it disabled or enabled).
I don't think there are much uses cases where English wouldn't be available in the source data, since you'd have removed the "en" entry from your D6/D7 {languages} table then, and in the process probably corrupted your whole D6/D7 installation.

The migration experience might be a bit weird for people migrating a site that's for example only in French (French enabled, core English disabled). Their D8 site in French will suddenly also have English in their list of languages after migration, even though they might not expect it. But then it should only be a matter of removing English again, once they verified they truly have no English content on their site.

So the current state of things as far as I can see, with the patch in #6:
All languages (enabled & disabled) from D6/D7 get migrated to D8. Migrated content gets assigned the correct language. There is no data loss.
Possibly the "Problem/Motivation" for this issue needs to be altered, since it mentions migrating *enabled* languages, whereas the current patch migrates *all* languages.

Correct me if I'm wrong.

@svendecabooter The issue of not having English arise if you install D8 in another language. You won't have English on your site, but you will have content which may be English. That's why we decided to install English anyway.

For more info on disabled language, check Gábor's comment on https://www.drupal.org/node/2166875#comment-8706173

With the patch in #6, you will have English installed in D8 after performing a migration, even if your D6 site has it disabled, and your D8 site is installed in another language (and hasn't activated English afterwards).

So all that's left to do is perhaps write additional tests for this?

More feedback welcome on this, so we can unblock the other i18n-migrate issues

How can it be that English is in your site? I don't see that in the patch at #6.

Because the English language (amongst potential others) gets selected by this query:

  public function query() {
    return $this->select('languages')->fields('languages');
  }

Or is there a situation where English would need to be migrated, but not be in the D6 / D7 {languages} table?

I tried with both d6 and d7 and you are right, English is there. I don't know why we added that there then.
So +1 to RTBC.

Great to see that both D6/D7 code is the same here. :)

One thing we talked about was that the tests are located in the D6 directory, and that code will eventually go away. However, it should be possible at that time to do a find/replace for D6 MigrateBase to D7 MigrateBase if we detect that a migration uses both.

Committed and pushed to 8.0.x. Thanks!

Yay, good first step :)