Apart from the theoretical problem of how to incorporate external dictionaries in a mathematically sound way into a statistical framework for machine translation (Brown, Della Pietra, Della Pietra, and Goldsmith 1993) there are also some pragmatic difficulties: As discussed in Section 2.2, one of the disadvantages of these conventional dictionaries as compared to full bilingual corpora is that their entries typically contain single words or short phrases on each language side.
it is not possible to distinguish among the translations for different readings of a word.