Here you'll find a list of a bunch of religious institutions in Kassel, including several Catholic Churches. I'd probably just contact the nearest one and ask. The Parish Priest should be able to tell you if there's an English speaking Catholic community in the area, or even just a Priest who speaks good English that you can go to for Confession.
Apparently St. Bonifatius is planning a special service on Sunday, Nov 19th with a visiting Abbot from Cameroon. The service will be in German, English and French. So I assume that there's at least one Priest around who can communicate well in English. More info
here. (in German)
Sorry I can't be of more help.
Vera