Limit a collection (and its pages) to specific languages #318

Feature request

Allow a collection (e.g. Blog) to be restricted to one or more specific languages — for example, only the default/main language — and hidden for all other locales.

Problem

On multilingual sites, every collection is currently available across all locales. Some content doesn't need (or shouldn't have) translations. A user may keep their Blog only in the main language, but right now there's no way to scope a collection to a subset of languages. This leads to:

• Empty or fallback (untranslated) blog content surfacing under non-default locales
• Localized collection/dynamic-page URLs existing for languages that have no real translated content
• Those URLs being included in the sitemap + hreflang clusters, which can create duplicate/thin-content signals for search engines

Proposed solution

Add a per-collection language scope setting, e.g.:

• Available in: All languages (default) / Selected languages
• When "Selected languages" is chosen, pick which locales the collection is enabled for (at minimum, "default language only")

When a collection is restricted, for any excluded locale:

• Hide the collection's dynamic pages and CMS-bound content from that locale
• Return 404 (or redirect to the default locale) for the excluded localized URLs
• Exclude those URLs from sitemap.xml and from hreflang alternates
• Optionally hide the collection from the language switcher context where it doesn't apply

Open questions

• Should this be configured at the collection level, the dynamic-page level, or both?
• For excluded locales, is the preferred behavior a 404 or a redirect to the default-locale equivalent?
• Should individual collection items also be scopeable per language, or only the whole collection?

Context

Requested by a user who wants to keep their Blog in the main language only and hide it for other languages.