Content localization: the same entry in several languages

There is no way to say "this is the Tagalog version of that post". Anyone needing two languages currently models it by hand with duplicate entries and a convention, and the delivery API cannot help them.

**Scope**

- A locale on an entry, and a link between translations of the same thing
- Delivery honours `?locale=` and falls back to a default when a translation is missing
- The admin can see which entries are missing a translation, which is the workflow that actually matters

**Decide before building**

- Whether locale is per entry or per field. Per entry is simpler and covers most cases; per field is what you eventually want for a page with a shared hero image.
- Fallback behaviour: return the default locale, or 404? Different sites want different answers, so it may need to be configurable.

Large enough to be worth agreeing the shape in this issue before writing code.