The "language_select" field widget is part of Core, but its config schema is in a module

Problem/Motivation

Core itself defines a “Language select” field widget (\Drupal\Core\Field\Plugin\Field\FieldWidget\LanguageSelectWidget). However, the config schema definition for that widget (added in #2827784-34: LanguageSelectWidget should have options for which language types to show) is part of The Language module, in core/modules/language/config/schema/language.schema.yml.

Therefore, it is possible to use the “Language select” field widget without having its config schema definition available.

Steps to reproduce

Define a new content entity type where the langcode field is set to have a configurable form display with default type language_select. Any test without the language module enabled will fail with an error like this:

Drupal\Core\Config\Schema\SchemaIncompleteException: Schema errors for core.entity_form_display.simplenews_subscriber.simplenews_subscriber.default with the following errors: core.entity_form_display.simplenews_subscriber.simplenews_subscriber.default:content.langcode.settings.include_locked missing schema

Proposed resolution

Simply move the config schema definition to Core, where it belongs.

Remaining tasks

• Review patch
• Probably write a test?

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

None needed, I’d guess?