A Study on Multilingual Transfer Learning in Neural Machine Translation - Finding the Balance Between Languages.

Transfer learning is an interesting approach to tackle the low resource languages machine translation problem. Transfer learning, as a machine learning algorithm, requires to make several choices such as selecting the training data and more particularly language pairs and their available quantity and quality. Other important choices must be made during the preprocessing step, like selecting data to learn subword units, the subsequent model’s vocabulary. It is still unclear how to optimize this transfer. In this paper, we analyse the impact of such early choices on the performance of the systems. We show that systems performance are depending on quantity of available data and proximity of the involved languages as well as the protocol used to determined the subword units model and consequently the vocabulary. We also propose a multilingual approach to transfer learning involving a universal encoder. This multilingual approach is comparable to a multi-source transfer learning setup where the system learns from multiple languages before the transfer. We analyse subword units distribution across different languages and show that, once again, preprocessing choices impact systems overall performance.