The description for Activate Session contains the following paragraph:

When a Client supplies a list of locale ids in the request, each locale id is required to contain the language component. It may optionally contain the <country/region> component. When the Server returns the response, it also may return both the language and the country/region or just the language as its default locale id.

The wording starting with "When the Server returns the response" is wrong and needs to be fixed. The response of ActivateSession does not return any locale information.