I know of no way to hacking Windows Speech Recognition in this manner. I'd have suggested the 2-user-account-solution-with-switching, but you've already thought of that.
I would not invest too much time in hacking Windows in that way. It sounds tedious and time consuming and will possibly break other things. (Though it might work through the registry, if you have time to experiment.)
2 alternatives:
- Buy a speech recognition app in the Windows store. There are various SpeechTrans products that allow you to dictate the text and then paste it where you need it. Maybe there are others but a quick search did not bring up anything. You can't dictate outside the app, but it's better than nothing.
- If speech recognition is so important to you, I recommend you ditch Windows RT and get yourself a computer with a full version of Windows 8. There you can install Dragon NaturallySpeaking or Dragon Notes* in English and use that in any language you purchase without regard to your OS language.
*Notes is a 20$ application that will let you dictate in English in Windows 8, though it does not work in Windows RT. More info here: http://www.nuance.com/dragon/dragon-notes/index.htm; Compatibility info under "Tech Specs".