toggleGoogleResultPageLang.user.js

// ==UserScript==
// @name           Toggle Google results page language
// @namespace      http://bitmap.dyndns.org/
// @description    Japanese <-> English
// @include        http://*.google.*/search?*
// ==/UserScript==
(function() {
  window.__toggleGoogleLinkClickFlag = false;
  document.addEventListener('mousedown', function(e) {
    window.__toggleGoogleLinkClickFlag = ( e.target.tagName == 'A' );
  }, false);

  document.addEventListener('dblclick', function(e) {
    if ( window.__toggleGoogleLinkClickFlag ) return;

    //thanks to http://d.hatena.ne.jp/ymorimo/20070719/1184847240
    var r=RegExp,l=location;
    if (l.href.match(new r("(http://[^/]+\\.google\\.[^/]+/search)(\\?.+)"))) {
      var u=r.$1,p=r.$2,q=(p.match(/[&\?]q=([^&]*)/))[1];
      if (q) {
        if (p.match(/[&\?]hl=([^&]*)/)) var hl=r.$1;
        var nhl=(hl=="en"?"ja":"en");
        l.href=u+"?q="+q+"&hl="+nhl+"&lr="+{"en":"","ja":"lang_ja"}[nhl];
      }
    }
  }, false);
})();