Massively multilingual transformers pretrained with language modeling objectives (e.g., mBERT, XLM-R) have become a de facto default transfer paradigm for zero-shot cross-lingual transfer in NLP, offering unmatched transfer performance. Current downstream evaluations, however, verify their efficacy predominantly in transfer settings involving languages with sufficient amounts of pretraining data, and with lexically and typologically close languages. In this work, we analyze their limitations and show that cross-lingual transfer via massively multilingual transformers, much like transfer via cross-lingual word embeddings, is substantially less effective in resource-lean scenarios and for distant languages. Our experiments, encompassing three lower-level tasks (POS tagging, dependency parsing, NER), as well as two high-level semantic tasks (NLI, QA), empirically correlate transfer performance with linguistic similarity between the source and target languages, but also with the size of pretraining corpora of target languages. We also demonstrate a surprising effectiveness of inexpensive few-shot transfer (i.e., fine-tuning on a few target-language instances after fine-tuning in the source) across the board. This suggests that additional research efforts should be invested to reach beyond the limiting zero-shot conditions.

分析了massively multilingual transformers在零射击跨语言场景中的局限性，并表明在资源匮乏和对比较遥远语言的情况下通过多语言转换的跨语言转移实际上不太有效。通过几个低级和高级自然语言处理任务的实验，确立了源语言和目标语言之间的语言相似度以及目标语言的预训练语料库的大小与转移性能之间的相关性。另外，研究表明通过在源语言上调整细节，再在目标语言上进行少量微调的few-shot transfer在多语言转换中十分有效。

从零到英雄：多语言Transformer的零-shot跨语言转移限制