It was not clear if this was being done by the browser /server or your page JS. And it was not clear you wanted to transition away from index.de.html or index.en.html .
So from what I am understanding, you want the server to point the correct scene in a Hype Page.
You may want to look at
This should allow you to point to individual scenes on a single Hype page and solve the browser back button. I refer to anchors.