Establishing Interlingua in Multilingual Language Models

Large multilingual language models show remarkable zero-shot cross-lingual transfer performance on a range of tasks. Follow-up works hypothesized that these models internally project representations of different languages into a shared interlingual space. However, they produced contradictory results. In this paper, we correct the famous prior work claiming that "BERT is not an Interlingua" and show that with the proper choice of sentence representation different languages actually do converge to a shared space in such language models. Furthermore, we demonstrate that this convergence pattern is robust across four measures of correlation similarity and six mBERT-like models. We then extend our analysis to 28 diverse languages and find that the interlingual space exhibits a particular structure similar to the linguistic relatedness of languages. We also highlight a few outlier languages that seem to fail to converge to the shared space. The code for replicating our results is available at the following URL: https://github.com/maksym-del/interlingua.