To Translate or Not to Translate: A Systematic Investigation of Translation-Based Cross-Lingual Transfer to Low-Resource Languages

Perfect machine translation (MT) would render cross-lingual transfer (XLT) by means of multilingual language models (LMs) superfluous. Given, on one hand, the large body of work on improving XLT with multilingual LMs and, on the other hand, recent advances in massively multilingual MT, in this work, we systematically evaluate existing and propose new translation-based XLT approaches for transfer to low-resource languages. We show that all translation-based approaches dramatically outperform zero-shot XLT with multilingual LMs, rendering the approach that combines the round-trip translation of the source-language training data with the translation of the target-language test instances the most effective. We next show that one can obtain further empirical gains by adding reliable translations to other high-resource languages to the training data. Moreover, we propose an effective translation-based XLT strategy even for languages not supported by the MT system. Finally, we show that model selection for XLT based on target-language validation data obtained with MT outperforms model selection based on the source-language data. We hope that our findings encourage adoption of more robust translation-based baselines in XLT research.