mT5: A Massively Multilingual Pre-trained Text-to-Text Transformer
Linting XueNoah ConstantAdam RobertsMihir KaleRami Al-RfouAditya SiddhantAditya BaruaColin Raffel
Presents mT5, a multilingual text-to-text transformer trained across 101 languages that establishes state-of-the-art performance on cross-lingual benchmarks and provides a practical technique to prevent accidental translation in zero-shot generation.
The paper introduces mT5, a multilingual extension of the T5 text-to-text model, to overcome the English-only focus of most large language models. Roughly 80 percent of the global population does not speak English, so models trained solely on English data leave the majority of users without effective tools for core NLP tasks such as classification, named-entity recognition, and question answering.
The work set out to test whether the T5 recipe—unified text-to-text format, span-corruption pre-training, and large scale—could be applied directly to 101 languages while preserving strong performance. The authors built mC4, a cleaned Common Crawl corpus covering those languages, increased the vocabulary to 250,000 word pieces, and trained five model sizes ranging from 300 million to 13 billion parameters. They evaluated the models on the XTREME benchmark suite under zero-shot, translate-train, and in-language multitask conditions and introduced a lightweight remedy for “accidental translation,” in which a generative model produces part of its output in the wrong language.
The largest model, mT5-XXL, surpassed prior state-of-the-art results on five of the six XTREME tasks in the zero-shot setting and on all tasks when English training data plus machine translations were available. Performance gaps between zero-shot and fully supervised multilingual training narrowed sharply as model size grew, indicating that scale reduces the need for translated or gold target-language labels. The accidental-translation problem declined by more than 70 percent relative when a small fraction of the original mC4 pre-training task was mixed into fine-tuning. Ablations confirmed that a language-sampling exponent of 0.3 and a line-length filter each contributed measurable gains.
These outcomes show that a straightforward scaling of an existing English recipe can deliver competitive cross-lingual transfer without specialized parallel data or language-specific filtering. The public release of checkpoints and code therefore gives practitioners immediate access to a single model family that supports both classification and generation across many languages.
Further gains will likely require additional compute for even larger models, targeted data collection for the lowest-resource languages, and systematic study of inference-time constraints that keep generative outputs within legal spans. The main uncertainties are the high run-to-run variance observed on some tasks and the still-limited absolute performance on the scarcest languages.
- Paper: Exploring the Limits of Transfer Learning with a Unified Text-to-Text Transformer, Colin Raffel et al. (2020). Reading the original T5 paper provides the foundational encoder-decoder architecture, text-to-text formatting, and pretraining objectives that mT5 directly scales to multilingual settings.
- Paper: Attention Is All You Need, Ashish Vaswani et al. (2017). Understanding the original Transformer architecture is essential for following how sequence-to-sequence models are constructed and trained.
- Paper: ST-MoE: Designing Stable and Transferable Sparse Expert Models, Barret Zoph et al. (2022). This paper builds directly upon large-scale multilingual encoder-decoder models by introducing sparse mixture-of-experts variants to scale capacity efficiently.
