User profile fields in Drupal 6 are translated using the i18nprofile submodule of the i18n suite. We need to migrate these translations.
1. Establish a system to migrate configuration translations since this is the first issue that needs it. Drupal\migrate\Plugin\migrate\destination\EntityConfigBase is changed to take the language manager and the config factory in its constructor. Change import() and updateEntity() to support translations. Expand rollback() to support for rolling back config translations.
2. User profile field translations are stored within i18n_strings and locales_target tables connected by the lid key on Drupal 6. These profile values are fields in the user entity on Drupal 8. Migrate the title and description of the fields to Drupal 8.
3. Add tests.
Migrate field options translations (if field options themselves are migrated). This is to be handled in.
User interface changes
Constructor of Drupal\migrate\Plugin\migrate\destination\EntityConfigBase gets two new arguments. As per https://www.drupal.org/core/d8-bc-policy the constructor of a plugin is considered internal and thus does not explicitly count as an API change.
Data model changes