When browsing the Internet one of the things I do most often is found on the Wikipedia not know or things that I have not entirely clear. It happens that after a clean install of Ubuntu I find that the search tool built into Firefox is configured for English Wikipedia. A lack of a better solution I do is edit the file wikipedia.xml .
For this test version of Firefox that I have installed is the 3.0.8 on my Ubuntu 8.10 but I guess it will work with other adapting it to the correct directory.
The first thing we do is open a terminal and type out in the

sudo gedit /usr/lib/firefox-3.0.8/searchplugins/wikipedia.xml

Once you write the correct password will open as root the file that describes how to access Wikipedia in English. Enter the text will look for the address of the server that will be something like Wikipedia http://en.wikipedia.org . In my case I get three times. Just going to change for us to be subdomio http://es.wikipedia.org three times. That is the "in" change for "is".

If you want to customize it a little more we can change where it says

Wikipedia (en)
Wikipedia, the free encyclopedia

by

Wikipedia (es)
Wikipedia, la enciclopedia libre

After this will save your changes. We close gedit and restart Firefox. From now on the search be conducted in Spanish Wikipedia.