Hmm, is there any reason to use Siri over Google's own voice recognition/text to speech stacks? I really doubt there's any significant difference between them in that regard at this point.
I don't think this is a problem worth attempting for anyone except Google at this point - they're pretty much there. Translate has a conversation mode, you choose the languages, and tap which language you're speaking in, it translates to + speaks in the other pretty much instantly. They've just added always listening to the Nexus 5's home screen and to Google Now.
Putting those two together, they pretty much have this working - 'just' add always listening to conversation mode for hardware that will support it (Siri can't do this by the way as far as I know), and then detect which language is being spoken (I suspect Google already have algorithms to get a language for webpages) and translate+speak back as normal.