Since the deciphering model described by Dou and Knight (2012) does not consider word reordering, it needs to decipher the bigram into "nations united" in order to get the right word translations "naciones"→"nations" and "unidas"→"united".
the English language model used for decipherment is built from English adjacent bigrams, so it strongly disprefers "nations united" and is not likely to produce a sensible decipherment for "naciones unidas".