Exploring the Relationship between Alignment and Cross-lingual Transfer
  in Multilingual Transformers
Félix Gaschi Patricio Cerda Parisa Rastin Yannick Toussaint
Without any explicit cross-lingual training data, multilingual language
models can achieve cross-lingual transfer. One common way to improve this
transfer is to perform realignment steps before fine-tuning, i.e., to train the
model to build similar representations for pairs of words from translated
sentences. But such realignment methods were found to not always improve
results across languages and tasks, which raises the question of whether
aligned representations are truly beneficial for cross-lingual transfer. We
provide evidence that alignment is actually significantly correlated with
cross-lingual transfer across languages, models and random seeds. We show that
fine-tuning can have a significant impact on alignment, depending mainly on the
downstream task and the model. Finally, we show that realignment can, in some
instances, improve cross-lingual transfer, and we identify conditions in which
realignment methods provide significant improvements. Namely, we find that
realignment works better on tasks for which alignment is correlated with
cross-lingual transfer when generalizing to a distant language and with smaller
models, as well as when using a bilingual dictionary rather than FastAlign to
extract realignment pairs. For example, for POS-tagging, between English and
Arabic, realignment can bring a +15.8 accuracy improvement on distilmBERT, even
outperforming XLM-R Large by 1.7. We thus advocate for further research on
realignment methods for smaller multilingual models as an alternative to
scaling.