Enhancement: Subscription - Support for multi-lingual site and subscribers

### Impacted plugin

Jetpack

### What

A way to send posts to a certain set of language subscribers.

### How

"Multilingual — Core implementation for Multilingual sites" is part of the [long term Gutenberg roadmap](https://wordpress.org/about/roadmap/), so it may be worth tracking interest for this request.

An idea, for multi-lingual sites:
- Site owner could set a choice of languages their content is available in
- When subscribing, Subscribers would have to select the language in which they want to receive the content
- Site owner would then be able to send posts to a particular language list

Requested in https://wordpress.org/support/topic/subscribers-useless-for-multilingual-websites/

No files, tests, or entry points are named. Start by reviewing Jetpack's subscription implementation and the linked Gutenberg multilingual roadmap, then compare the requested language-selection and per-language delivery behavior with the current design. Done would require an agreed implementation scope and support for language-specific subscriber lists.

Written by the indexing model from the issue text.