Welcome to the forums. Please be mindful to use the CODE tags and indent for readability when posting code.
What you can do is run through each link in the for loop and test for a class as you go, e.g.
Code:
window.onload = function () {
var links= document.getElementsByTagName('a');
for (var i = 0, l= links.length; i<l; i++) {
if ( links[i].className == 'changelink' ) {
links[i].href += '&layout=jp';
}
}
};
I've altered it slightly, using a DOM method to get all the anchor tag elements, then checking each one for a specific className. The loop is slightly different as well, assigning the links length value once rather than evaluating it on each iteration.