[webmention] Issue: Section 3.2: no language support marked as i18n

r12a has just labeled an issue for https://github.com/w3c/webmention 
as "i18n":

== Section 3.2: no language support ==
[raised by Addison Phillips, discussed in i18n telecon]

https://www.w3.org/TR/webmention/#receiving-webmentions

Section 3.2 say (in part):

>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.

See https://github.com/w3c/webmention/issues/57