extlink.setting must specify a language code, because it contains translatable values

Problem/Motivation

extlink.settings contains translatable values, so it must have a language code. https://www.drupal.org/node/3427629

Steps to reproduce

This is checked by LangcodeRequiredIfTranslatableValuesConstraint.

\Drupal::service('config.typed')->createFromNameAndData(
    'extlink.settings',
    \Drupal::service('config.factory')->get('extlink.settings')->get()
)->validate();

Proposed resolution

Add langcode to the default configuration and add it to existing config in an update hook.

Remaining tasks

User interface changes

API changes

Data model changes