(dev/translation#78) I18nSubscriber - Allow partial/negotiated locales

(dev/translation#78) I18nSubscriber - Allow partial/negotiated locales

Definitions:

* A fully-supported locale is valid in all localization services (eg
  `ts()`, `Civi::format()`, `$dbLocale`).
* A "negotiated" or "mixed" locale can be used for communication, but
  it is not fully supported by all other layers. Consequently, it
  requires some kind of fallback or substitution.

Before:

* When an API call requests an alternate language (v3's `option.language` or
  v4's `setLanguage()`), it only activates fully-supported locales.
* Specifically, it validates against the multilingual configuration -- those
  are fully supported locales.) It otherwise ignores the alternate language.

After:

* When an API call requests an alternate language, it cacn activate
  fully-supported locales as well as mixed locales.
* Specifically, it asks `Locale::negotiate(...)` to examine the local
  configuration/resources and determine the effective locale.