Improving Zero-Shot Cross-Lingual Transfer Learning via Robust Training

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In recent years, pre-trained multilingual language models, such as multilingual BERT and XLM-R, exhibit good performance on zeroshot cross-lingual transfer learning. However, since their multilingual contextual embedding spaces for different languages are not perfectly aligned, the difference between representations of different languages might cause zero-shot cross-lingual transfer failed in some cases. In this work, we draw connections between those failed cases and adversarial examples. We then propose to use robust training methods to train a robust model that can tolerate some noise in input embeddings. We study two widely used robust training methods: adversarial training and randomized smoothing. The experimental results demonstrate that robust training can improve zero-shot crosslingual transfer for text classification. The performance improvements become significant when the distance between the source language and the target language increases.