  While neural networks have been used extensively to make substantial progress
in the machine translation task, they are known for being heavily dependent on
the availability of large amounts of training data. Recent efforts have tried
to alleviate the data sparsity problem by augmenting the training data using
different strategies, such as back-translation. Along with the data scarcity,
the out-of-vocabulary words, mostly entities and terminological expressions,
pose a difficult challenge to Neural Machine Translation systems. In this
paper, we hypothesize that knowledge graphs enhance the semantic feature
extraction of neural models, thus optimizing the translation of entities and
terminological expressions in texts and consequently leading to a better
translation quality. We hence investigate two different strategies for
incorporating knowledge graphs into neural models without modifying the neural
network architectures. We also examine the effectiveness of our augmentation
method to recurrent and non-recurrent (self-attentional) neural architectures.
Our knowledge graph augmented neural translation model, dubbed KG-NMT, achieves
significant and consistent improvements of +3 BLEU, METEOR and chrF3 on average
on the newstest datasets between 2014 and 2018 for WMT English-German
translation task.
