Cross-Lingual Ability of Multilingual BERT: An Empirical Study

Recent work has exhibited the surprising cross-lingual abi lities of multilingual BERT ( M-BERT) - surprising since it is trained without any cross-lingual objective and with no aligned data. In this work, we provide a compr ehensive study of the contribution of different components in M-BERT to its cross-lingual ability. The experimental study is done in the context of three typologically different languages - Spani sh, Hindi, and Russian - and using two conceptually different NLP tasks, textual en tailment and named entity recognition. Among our key conclusions is the fact th at the lexical overlap between languages plays a negligible role in the cross-ling ual success, while the depth of the network is an integral part of it. Embeddings of natural language text via unsupervised learn ing, coupled with sufficient supervised training data, have been ubiquitous in NLP in recent years an d have shown success in a wide range of monolingual NLP tasks, mostly in English. Training models f or other languages have been shown more difficult, and recent approaches relied on bilingual em beddings that allowed the transfer of supervision in high resource languages like English to mode ls in lower resource languages; however, inducing these bilingual embeddings required some level of supervision (Upadhyay et al., 2016). Not only the model is contextual, but its training also requires no supervisio n - no alignment between the languages is done. Nevertheless, and despite being trained with no exp licit cross-lingual objective, M-BERT produces a representation that seems to generalize well acr oss languages for a variety of downstream tasks (Wu & Dredze, 2019). In this work, we attempt to develop an understanding of the su ccess of M-BERT.