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Towards Integrated Classification Lexicon for Handling Unknown Words in Chinese-Vietnamese Neural Machine Translation

In Neural Machine Translation (NMT), due to the limitations of the vocabulary, unknown words cannot be translated properly, which brings suboptimal performance of the translation system. For resource-scarce NMT that have small-scale training corpus, the effect is amplified. The traditional approach of amplifying the scale of the corpus is not applicable, because the parallel corpus is difficult to obtain in a resource-scarce setting; however, it is easy to obtain and utilize external knowledge, bilingual lexicon, and other resources. Therefore, we propose classification lexicon approach for processing unknown words in the Chinese-Vietnamese NMT task. Specifically, three types of unknown Chinese-Vietnamese words are classified and their corresponding classification lexicon are constructed by word alignment, Wikipedia extraction, and rule-based methods, respectively. After translation, the unknown words are restored by lexicon for post-processing. Experiment results on Chinese-Vietnamese, English-Vietnamese, and Mongolian-Chinese translations show that our approach significantly improves the accuracy and the performance of NMT especially in a resource-scarce setting.