AIP-193: Update the rationale around LocalizedMessage

Note: this removes the idea that LocalizedMessage is for "end users" as a distinction between that and Status.message being for developers. If we still want that distinction, we should talk about it more and clarify it - in particular, if we're using LocalizedMessage as an alternative for Status.message where the latter can't be changed, they can't really cater for different audiences.

aip/general/0193.md

@@ -392,28 +392,28 @@ variable information, so there needs to be machine-readable component of
 *every* error response that enables clients to use such information
 programmatically.
 
-### LocalizedMessage
+### Including LocalizedMessage
 
 `LocalizedMessage` was selected as the location to present alternate
-error messages. This is desirable when clients need to display a crafted
-error message directly to end users. `LocalizedMessage` can be used with
-a static `locale`.  This may seem misleading, but it allows the service
-to eventually localize without having to duplicate or move the error
-message, which would be a backwards incompatible change.
-
-Reasons to present the same error message in both locations include the following:
-
-- The service plans to localize either immediately or in the near future. See,
-  "[Localization](#localization)".
-- This practice enables clients to find an error message consistently in one
-  location, `LocalizedMessaage.message`, across all methods of the API Service.
-
-Reasons to present an error message in `LocalizedMessage.message` that
-differs from `Status.message` include the following:
-
-- The service requires an end-user-facing error message that differs
-  from the "debug message".
-- Ongoing, iterative error message improvements are expected.
+error messages. While `LocalizedMessage` **may** use a locale specified
+in the request, a service **may** provide a `LocalizedMessage` even without
+a user-specified locale, typically to provide a better error message in
+[situations where `Status.message` cannot be changed](updating-statusmessage).
+Where the locale is not specified by the user, it **should** be `en-US`
+(US English).
+
+A service **may** include `LocalizedMessage` even when the same message is
+provided in `Status.message` and when localization into a user-specified locale
+is not supported. Reasons for this include:
+
+- An intention to support user-specified localization in the near future, allowing
+  clients to consistently use `LocalizedMessage` and not change their error-reporting
+  code when the functionality is introduced.
+- Consistency across all RPCs within a service: if some RPCs include
+  `LocalizedMessage` and some only use `Status.message` for error messages, clients
+  have to be aware of which RPCs will do what, or implement a fall-back mechanism.
+  Providing `LocalizedMessage` on all RPCs allows simple and consistent client code
+  to be written.
 
 ### Updating Status.message