It was me who made the fan translation of this addon.
It's not that difficult to get it working, I made a helper app to install the German addon on the English game, so it's as easy as it gets: run the helper app, insert the addon cd when asked to do so and it installs. A second installer fixes the translations to English.
You can read about the procedures and get the downloads in the UBIsoft forums: http://forums.ubi.com/showthread.php/203258-Vikings-add-on-How-to-Install-Play-on-English-Version?p=1832447&viewfull=1#post1832447
I hate to say so, but if you want it, probably your best bet is to get it second hand from amazon.de(http://www.amazon.de/gp/offer-listing/B000MCHERS/ref=dp_olp_used?ie=UTF8&condition=used
) or illegally download the German addon cd
I'd like to see gog offer it, and I also offered GoG to use my translations for free, but never got a reply to that.