RT-SFT: Text Style Transfer from Non-Parallel Corpora by Roundtrip Translation

Ruoxi Liu, Philipp Koehn


Abstract
Text style transfer (TST) is naturally a supervised task — rewrite a sentence in a target style while preserving its meaning — yet the parallel corpora that supervision requires exist for only a handful of style domains. A common workaround is to normalize an input into a style-agnostic intermediate and then stylize it into the target style, but the normalizer is typically a lightweight, task-specific paraphraser applied only at test time, feeding a correspondingly small stylizer. We observe that a style-stripping normalizer already exists at scale: neural MT systems trained on hundreds of millions of general-domain sentence pairs preserve content while regressing toward generic phrasing, so roundtrip translation through a pivot language strips stylistic signal without any task-specific training. This turns normalization from an inference-time patch into a data-generation tool. Roundtrip-translating a monolingual in-style corpus yields a pseudo-parallel corpus on which we LoRA-finetune an instruction-tuned LLM as the stylizer (RT-SFT); applying the same normalizer to test queries keeps that stylizer in-distribution. We show that across four style domains, RT-SFT outperforms state-of-the-art methods, such as few-shot in-context learning, by considerable margins. We also report on effective retrieval augmentation methods for expert style domains with strict terminology and naming conventions.
Anthology ID:
2026.wmt-main.23
Volume:
Proceedings of the Eleventh Conference on Machine Translation
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Barry Haddow, Tom Kocmi, Philipp Koehn, Christof Monz
Venues:
WMT | WS
SIG:
SIGMT
Publisher:
Association for Computational Linguistics
Note:
Pages:
398–411
Language:
URL:
https://aclanthology.org/2026.wmt-main.23/
DOI:
Bibkey:
Cite (ACL):
Ruoxi Liu and Philipp Koehn. 2026. RT-SFT: Text Style Transfer from Non-Parallel Corpora by Roundtrip Translation. In Proceedings of the Eleventh Conference on Machine Translation, pages 398–411, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
RT-SFT: Text Style Transfer from Non-Parallel Corpora by Roundtrip Translation (Liu & Koehn, WMT 2026)
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PDF:
https://aclanthology.org/2026.wmt-main.23.pdf