I couldn't see if anyone had posted this before, so here goes. This works OK for me, and is easy to do, but I take no responsibility for any damage you do to your PC or your installation!
It isn't perfect as I'll explain below, but it is certainly good enough to play. I am really dissapointed that UBISoft didn't release this in English. I am sure there is a market for it.

Steps
1. Install Settlers II 10th Anniversary (if you haven't already!)
2. Apply the 11757 patch (s2patch11757.exe) (if you haven't already!)
3. Make a copy of the following folder

<Your Install Path>\The Settlers II - 10th Anniversary\data\language\gb

and its contents. e.g. copy it to gb_copy
4. Rename the original 'gb' folder to 'de'
5. Using a registry editor (eg regedit) navigate to the key

HKEY_LOCAL_MACHINE\SOFTWARE\Ubisoft\Funatics\S2TNG

There you will find a String Value called Language. It will read 1033 (English). Change it to 1031 (German).
6. Install the Vikings Add-on from the CD, wait for it to finish.
7. Copy the contents of the previously backed up gb folder (

<Your Install Path>\The Settlers II - 10th Anniversary\data\language\gb_copy

from the example above) into the de folder, overwriting all files when asked.
8. Change the registry key back to 1033.
9. Start the Vikings add-on and play...

There are a few issues.
1. The mission briefings are in German - not a big problem, as each mission is quite simple to grasp.
2. There are some great new features added to the game including notification when you are out of resources, announcements on tool production (so you don't end up with a million scythes ) and battle summaries (e.g. attack repelled). These are also in German, but a quick trip to Babelfish allowed me to translate them all. The resource names are in English though.
3. There are some other bits and bobs still in German but all the screens, menus, buildings etc etc are in English so it's pretty simple to play.

Some translations
Soldatennachschub ist knapp - Soldier supply is limited
Rohstoffe sind knapp - Raw materials are limited
Warenstau an fahne - Congestion at flag (very useful!)
Werkzeug hergestellt - Tool manufactured
Angriffe abgewehrt - Attacks repelled

Hope this helps someone. It's keeping me going until Settlers 6 ROAE (which was a disappointment )

This message has been edited. Last edited by: Settled,

Nice one - to post this I hope Settlers 6 is all you are waiting for.

I might try it when I have time. Did not know about that patch number so gone and got it and saved it for possible future use.

but like you still very annoyed at Ubisucks for not telling the truth and not releasing it here.

You Sir are a god damn genius. I love you...

There is one more thing to mention though. Some of you may be wondering -as I did throught the night- where the heck is the new random map generator that should come with the add-on.
After some hours of googling(which gave me no results), thinking and trying, at last I could do something that worked out:
It happens that telling the registry that you have the eng version, the map editor will use the Editor_English.dll, that is the dll of the earlier version. The add-on installed the Editor_German.dll (or editor_polish.dll, or whatever).
So if you want whatever new features the new add-on's map editor brings, such as the long awaited random map generator (which still sucks imo), all you have to do is to:
rename <install-path>\bin\Editor_English.dll to something else or delete it.
This will make the exe look for a dll of another language present in the directory.

I forgot to thank Settled for the nice discovery, you saved my day man.

Thanks for posting this very detailed guide!
I was actually thinking of buying this expansion but wasn't sure if the languages are compatible.