alert(_("Please select only one ")+"[% label_element %]"+_(" to")+op+"."); In addition to label_element and op being untranslatable, the " to" -string needs a space after it - otherwise we get messages like "Please select only one layout toedit."
And a few lines below that: alert(_("Please select a ")+"[% label_element %].");
This was fixed on Bug 11365