Sanity: Default language should always come from language schema

[dtaylorbrown]

At the moment we can detect the default language from the schema in sanity.config.ts but ideally we also need to be able to do this within the custom desk structure where we currently have:

const defaultLanguage = {
  id: 'en-US', title: 'English (US)', default: true
};

This could possibly cause some issues if the default language were to ever change in the schema

Acceptance criteria

• [ ] The default language should be retreived by the default langauge set in schema

[umxr]

This might be superseded by #39