Wow, not much time to prepare. This may be a disaster in the making.

I thought at first that the free Goodwill Guide service might be helpful for you but it seems they are only available in Hokkaido at Furano Ski Area and the city of Hakodate. I'm surprised at that as they are everywhere else in Japan and Sapporo has a university, which is where most of the volunteer guides are from.

There is a professional interpreting service but their rate is about $250.00 for a day. http://zls.ueuo.com/page1/index.html

Other than that, I am out of ideas. Perhaps your relatives in Sapporo have a friend who speaks English and can help.