LanguageItem lacks a schema for its default values

Problem/Motivation

We're missing a schema for field.value.language which means we cannot have a default value for a LanguageItem field type that passes config schema checking

Steps to reproduce

Add a test that creates a LanguageItem field type with a default value when strict schema checking is on.
See the test fail config schema validation

Proposed resolution

Add a schema entry for field.value.language

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet