I just noticed that swiftirc nickserv sends the notice "Password accepted - you are now recognized" whereas Freenode nickserv sends "You are now identified for nickname". So even using english to determine success will not work reliably as the wording could vary from network to network.
mIRC could delay perform by several seconds after seeing the last nickserv notice. This would resolve the issue in your particular case, unless nickserv takes too long. This will be in the next version.