>The response body MAY contain content, in which case a human-readable
response is recommended.
There is no mention of language negotiation or language identification
here. The assumption appears to be that a wad of English is returned?
The example could include a `Content-Language` header or might allow
for other language identification in the body (complicated)
This is also applicable to at least 3.2.3 Error Responses as well.
> If the Webmention was not successful because of something the sender
did, it MUST return a 400 Bad Request status code and MAY include a
description of the error in the response body.