Boosting Unsupervised Machine Translation with Pseudo-Parallel Data

After the great advancements in machine translation (MT) quality brought by neural MT (NMT; Bahdanau et al., 2015; Vaswani et al., 2017) trained on millions of pre-translated sentence pairs, there came a realization that parallel data is expensive and surely not available for most language pairs in the world. Researchers started focusing their attention on methods leveraging monolingual data for machine translation (Sennrich et al., 2016b) and even explored the extreme scenario of training a translation system in a completely unsupervised way with no parallel data at all (Artetxe et al., 2018b; Lample et al., 2018a). The recent impressive progress in language modeling did not leave the area of machine translation intact. However, the translation capabilities of large language models such as the latest GPT models (Brown et al., 2020) are weak for underrepresented languages (Hendy et al., 2023) and unsupervised MT aimed at low-resource languages still deserves special attention. There are two ways to approach machine translation trained exclusively on monolingual data. In the absence of parallel texts, the monolingual training sentences can either be coupled with their synthetic counterparts which are automatically generated through back-translation (Artetxe et al., 2018b; Lample et al., 2018a), or with authentic counterparts which are automatically selected from existing monolingual texts to be as close translations as possible (Ruiter et al., 2019). Researchers have successfully explored both of these avenues with the conclusion that it is indeed possible to train a functional MT system on monolingual texts only. However, little attention has been paid to combining the two approaches together. In this paper, we work with the standard framework for training unsupervised MT but we incorporate an additional training step where sentence pairs mined from monolingual corpora are used to train the model with a standard supervised MT objective.