Here's the offending code on the site:

http://sv.wikipedia.org/wiki/MediaWiki:Common.js

// Sökfunktion med flera sökmotorer
/ Change Special:Search to use a radio menu *************

• Description: Change Special:Search to use a radio menu, with the default being
• the internal MediaWiki engine
• Created and maintained by: [[:fr:User:Pmartin]] */

if ((wgNamespaceNumber == -1) && (wgCanonicalSpecialPageName == "Search")) {
var searchEngines = {

	  mediawiki: {
	    ShortName: "Wikipedia-sök",
	    Template: "/w/index.php?search={searchTerms}"
	  },
	  yahoo: {
	    ShortName: "Yahoo!",
	    Template: "http://" + wgUserLanguage + ".search.yahoo.com/search?p={searchTerms}&vs=" + wgServer
	  },

[snip]

wgUserLanguage will be 'sv', 'en', 'fr', etc... whereas the hostnames vary, but seem to be using country codes:

• se.search.yahoo.com
• fr.search.yahoo.com
• de.search.yahoo.com
• jp.search.yahoo.com -> search.yahoo.jp
• us.search.yahoo.com

Language codes just don't work except in a few countries/language that happen to match up like fr and de. If the language code can't be appended, it should just head straight to the Swedish server.

Bad code was introduced April 6 2009:
http://sv.wikipedia.org/w/index.php?title=MediaWiki:Common.js&diff=8931878&oldid=8672428

I've fixed it there and dropped a note on-wiki to [[fr:User:Pmartin]] about the issue; the fix may need to be replicated on other sites. (Pascal wouldn't have noticed the bug if his UI language was set to French.)

Note that aggressive caching of site JavaScript means that not all end-users will see the fix immediately; it may take up to 30 days to ensure that all visitors are updated.