Language management supports adding a full UI language

[Odzen]

Replaces old issue #836.

Problem

Admins can manage language option records, but adding a new full UI language still requires developer work: locale JSON files, i18n supportedLngs, labels, and translation coverage.

Required behavior

• Define the difference between language option records and full UI languages.
• Provide an admin/dev workflow for adding a new UI language without ad hoc manual steps.
• Generate or export the translation file format expected by the localization process.
• Ensure the language appears in the UI only when required translations are available.

Acceptance criteria

• A documented workflow exists for adding a new supported UI language.
• New language code is added to i18n configuration through a repeatable process.
• Locale JSON coverage can be validated before enabling the language selector option.
• Existing language option CRUD remains separate from UI locale enablement.