Adapter layers are lightweight, learnable units inserted between transformer layers. Recent work explores using such layers for neural machine translation (NMT), to adapt pre-trained models to new domains or language pairs, training only a small set of parameters for each new setting (language pair or domain). In this work we study the compositionality of language and domain adapters in the context of Machine Translation. We aim to study, 1) parameter-efficient adaptation to multiple domains and languages simultaneously (full-resource scenario) and 2) cross-lingual transfer in domains where parallel data is unavailable for certain language pairs (partial-resource scenario). We find that in the partial resource scenario a naive combination of domain-specific and language-specific adapters often results in `catastrophic forgetting' of the missing languages. We study other ways to combine the adapters to alleviate this issue and maximize cross-lingual transfer. With our best adapter combinations, we obtain improvements of 3-4 BLEU on average for source languages that do not have in-domain data. For target languages without in-domain data, we achieve a similar improvement by combining adapters with back-translation. Supplementary material is available at https://tinyurl.com/r66stbxj

本文研究了在机器翻译领域中适配器的组成方法，旨在实现多领域和多语言（全资源场景）的参数效率适配，或在无法提供特定语言对的平行数据（部分资源场景）中的跨语言转移。适配器通常由语言特定的适配器和领域特定的适配器组成，但这篇论文发现了将两种适配器简单组合的方法往往会造成缺失语言的灾难性遗忘，因此提出了新的适配器组合方式以减轻这一问题，并最大程度地实现跨语言转移。通过此种最佳适配器组合方式，我们实现了在没有场内数据的源语言上平均提高3-4BLEU，而在没有场内数据的目标语言上与反向翻译相结合也达到了类似的改进。

多语言领域自适应翻译：通过适配器解耦语言和领域信息