No default value for User langcode when migrating D7 users with no language

Problem/Motivation

The d7_user.yml migration template migrates the language field even if it is empty, i.e.:

...
process:
  ...
  langcode: language
  preferred_langcode: language
  preferred_admin_langcode: language
  ...

As a result custom user fields (which do have a language) are not displayed correctly.

Proposed resolution

On my site the default langcode is en so I have used a hook_migration_create() to make the required change:

  if ($migration->id() == 'd7_user') {
    $process = $migration->getProcess();
    $process['langcode'] = array(
      'plugin' => 'default_value',
      'source' => 'language',
      'default_value' => 'en',
    );
    $process['preferred_langcode'] = array(
      'plugin' => 'default_value',
      'source' => 'language',
      'default_value' => 'en',
    );
    $process['preferred_admin_langcode'] = array(
      'plugin' => 'default_value',
      'source' => 'language',
      'default_value' => 'en',
    );
    $migration->setProcess($process);
  }

This is equivalent to make the following changes to the d7_user.yml migration template:

...
process:
  ...
  langcode:
    plugin: default_value
    source: language
    default_value: en
  preferred_langcode:
    plugin: default_value
    source: language
    default_value: en
  preferred_admin_langcode:
    plugin: default_value
    source: language
    default_value: en
  ...

I suggest a general version of this is required, using the default_langcode of the site as the default value for these three fields. I suspect this will require a custom MigrateProcessPlugin (unless it is possible to pass system.site.default_langcode into the default_value MigrateProcessPlugin).

Remaining tasks

Decide how to solve this
Solve it!

User interface changes

None

API changes

None

Data model changes

None