[DX][Intl] Improved API for access to language, locale and country codes and names

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12775
| License       | MIT
| Doc PR        | -

Clearer API for  access to language, locale and country codes.
Maybe explain what happens when the locale is `null`.