var url = document.URL; 
var front = (url.substring(0,url.lastIndexOf("\_")+1));
var cur_lang = (url.substring(url.lastIndexOf("\_")+1,url.lastIndexOf("\.")));
var to_change;

var languageMagic = function() {
    var new_lang;
    if (cur_lang === 'e') new_lang = 'f';
    if (cur_lang === 'f') new_lang = 'e';
    var switch_selector = '#navbar a[href$="index_' + new_lang + '.html"]';
    to_change = $$(switch_selector).first();
    if (to_change !== undefined) to_change.href = front + new_lang + '.html';
}