Robbed from count's post, here are the details for changing the language to English...(although it does not seem to change all the interface, I'll work an producing a little starter guide for the interface, unless anyone knows if one already exists?)

You can change the language choice with a quick mod to the client.

There is a file in the following folder C:\Program Files\Brettspielwelt\ (If you used the default installation) called brettspielwelt.prop

Open this file in notepad and scroll down until you see the entry Nation. Change it to

Nation = en

Save the file and reload the client. It should now run the client with an english interface.