Lexically Stable but Semantically Improvable: On the Editability of Machine Translation Drafts for LLM Post-Editing

Tianlin Xiao, WenHan Chao


Abstract
Large language models (LLMs) perform strongly in machine translation, but their effectiveness as translation refiners depends on the draft source. We compare direct translation, self-refinement, and cross-system refinement on English-to-Chinese WMT test sets. Sockeye drafts obtain higher lexical-overlap scores but lower COMET than the stronger Qwen3 drafts, and receive larger gains after Qwen3 refinement. Additional controls qualify this pattern. Qwen3-0.6B drafts obtain the largest raw gain but lower final quality, while Qwen3-1.7B drafts start near Sockeye in COMET yet benefit less. Cross-family GLM-4-9B-Chat drafts also improve with Qwen3-8B, but achieve smaller gains and lower final quality than refined Sockeye drafts. Descriptive bucket, common-support, and regression analyses suggest that initial quality alone is an incomplete account of these differences. xCOMET-XL score and span diagnostics further characterize the contrast with Qwen3-8B self-refinement. We frame translation editability as a property of the evaluated draft–editor pairing, assessed jointly through quality gain, improvement probability, degradation risk, and final quality.
Anthology ID:
2026.aacl-long.58
Volume:
Proceedings of the 15th International Joint Conference on Natural Language Processing and the 5th Conference of the Asia-Pacific Chapter of the Association for Computational Linguistics (Volume 1: Long Papers)
Month:
November
Year:
2026
Address:
Hengqin, China
Editors:
Yue Zhang, Ruifeng Xu, Ayu Purwarianti, Tong Rong, Jun Suzuki
Venues:
AACL | IJCNLP
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
1236–1259
Language:
URL:
https://aclanthology.org/2026.aacl-long.58/
DOI:
Bibkey:
Cite (ACL):
Tianlin Xiao and WenHan Chao. 2026. Lexically Stable but Semantically Improvable: On the Editability of Machine Translation Drafts for LLM Post-Editing. In Proceedings of the 15th International Joint Conference on Natural Language Processing and the 5th Conference of the Asia-Pacific Chapter of the Association for Computational Linguistics (Volume 1: Long Papers), pages 1236–1259, Hengqin, China. Association for Computational Linguistics.
Cite (Informal):
Lexically Stable but Semantically Improvable: On the Editability of Machine Translation Drafts for LLM Post-Editing (Xiao & Chao, AACL-IJCNLP 2026)
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PDF:
https://aclanthology.org/2026.aacl-long.58.pdf