function check() {
    var state = langs.map(function(){return this.checked}).get().indexOf(false) === -1;      // select all ?
    selectAll.prop('checked', state);
    (state ? deselectAllLabel : selectAllLabel).show();
    (!state ? deselectAllLabel : selectAllLabel).hide();
}

langs.click(check);

selectAll.click(function () {
    langs.prop('checked', selectAll.prop('checked'));
    check();
});

invertSelect.click(function () {
    langs.each(function(){this.checked = !this.checked;});
    check();
});