Pre-training Multilingual Neural Machine Translation by Leveraging Alignment Information
Summary
The paper asks whether one pre-trained translation model can act as a shared starting point that, after fine-tuning, beats models trained from scratch on any language pair. Earlier MT pre-training methods such as MASS and mBART rely on auto-encoding objectives, which differ from translation, and they mainly help low-resource pairs. The authors propose mRASP. It is a Transformer-large model trained jointly on 32 English-centric language pairs (64 directions) from public parallel data. Pre-training uses the same translation loss as fine-tuning. The key addition is Random Aligned Substitution (RAS): source words are randomly swapped for dictionary translations in other languages, producing code-switched inputs that push words with similar meanings closer together in representation space. The authors evaluate 42 translation directions, ranging from extremely low-resource to rich-resource pairs, plus pairs whose languages were absent from pre-training. mRASP consistently outperforms direct training, including on English-French. This suggests that parallel data from many languages, even small corpora, can help translation for well-resourced languages too. Code, data and models are released.
Key points
- Pre-training data. The corpus is called PC32 and contains about 197M sentence pairs. Sources include TED, WMT, Europarl, ParaCrawl, OpenSubtitles and QED. RAS uses the MUSE bilingual En-X dictionaries where they exist.
- Model setup. The model has a 6+6-layer encoder-decoder, dimension 1,024 and 16 attention heads. It uses GeLU activations, learned positional embeddings and a shared 32k BPE vocabulary. Language tokens such as
<en>and<fr>mark the source and target sides. - Low-resource and rich-resource gains. For pairs under 100k sentences, gains reach roughly +29 BLEU over direct training. On WMT14 English-French, mRASP reaches 44.3 BLEU.
- Unseen languages. For exotic pairs not seen in pre-training, gains range from +3.3 to +14.1 BLEU. Dutch-Portuguese reaches 14.1 BLEU with only about 12k pairs.
- Analysis. For En→De, fine-tuning on just 1K pairs yields about 24.5 BLEU. Fine-tuning on 1M pairs roughly matches a baseline trained on the full 4.5M. RAS also raises the cross-lingual cosine similarity of word embeddings.
Cited by
- Contrastive Learning for Many-to-many Multilingual Neural Machine Translation
- Multilingual Translation from Denoising Pre-Training
Cite
@inproceedings{lin2020pre,
title = {{Pre-training Multilingual Neural Machine Translation by Leveraging Alignment Information}},
author = {Lin, Zehui and Pan, Xiao and Wang, Mingxuan and Qiu, Xipeng and Feng, Jiangtao and Zhou, Hao and Li, Lei},
booktitle = {Proceedings of the 2020 Conference on Empirical Methods in Natural Language Processing (EMNLP)},
year = {2020},
doi = {10.18653/v1/2020.emnlp-main.210},
url = {https://doi.org/10.18653/v1/2020.emnlp-main.210}
}References
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