Here's what the spec says
This attribute specifies the language of the speech synthesis for the utterance, using a valid BCP 47 language tag. [BCP47] If unset it remains unset for getting in script, but will default to use the lang of the html document root element and associated hierachy. This default value is computed and used when the input request opens a connection to the recognition service.
I believe this was taken care of with
http://trac.webkit.org/changeset/142433 at least for the Mac.