Is there a way to change the language for Wikipedia searches in iBooks on iOS, without changing the language settings for the device?
Yes there is, although you have to modify the iPad's files, so you either have to jailbreak your iPad, then use an app called iFile (paid with a free trial) or openSSH (free but more complicated - I used iFile), or find some way to modify your files without a jailbreak.

An explanation

iOS apps store data on "property list" files. To localise an app they have a property list file that stores all the stuff in the app that they want to change, and what to change it to. The one we are interested in has a field dealing with the Wikipidea search URL, all we have to do is change that URL to go to the English Wikipidea instead of the Norwegian one.

Steps (note that I am using iFile)


Using your chosen file manager navigate to /var/mobile/Applications
You should see the names of your apps as well as some random characters, if you only see the random characters then click the settings button (bottom left), go to preferences and turn on "App names"
Scroll down until you see the "iBooks" folder
In the iBooks folder, click "iBooks.app"
Scroll down until you see the folder "no.lproj" - this is the Norwegian language folder
Click on the file "Localizable.strings" open it with the property list viewer - this is the main file that deals with localisation
Scroll down to near the bottom until you find the field called "Search_Wikipidea_URL", tap on it
You should see a text box with the URL "http://%2$@.wikipedia.org/wiki/%1$@", change it to "http://en.wikipedia.org/wiki/%1$@"
Click "Done", then exit iFile
That's it!


Now to test it go to iBooks. When you search Wikipidia it will show the English version. I tested this on my iPod (yes, I put my iPod into Norwegian!) and it worked, so if you have a problem and want me to post pictures for the steps then just leave a comment!