Dynamic Lagging using Stable-Prefix Training for Simultaneous Translation

Hieu Hoang, Amittai Axelrod, Matt Post


Abstract
In streaming simultaneous speech translation, the speech translation system is trained to learn a read-write policy that alternates between consuming source words and generating target ones. In a cascaded setting, the output from the speech recognizer is passed to a separate machine translation component, making it more difficult to learn such a policy. Approximations such as fixed wait-k strategies or target-suffix deletion can be employed, but these approaches do not provide the model with a streaming system’s flexibility to make contextual read-write decisions. This paper presents a training strategy for a cascaded machine translation system that enables it to dynamically decide how much of the growing source prefix to translate. We achieve this by fine-tuning a large language model (Qwen3-8B) on stable prefixes of the training data, which are produced by pairing every source sentence prefix in the training data with the longest translation of that prefix that is shared with the full source sentence translation. We fine-tune variants of the model on different subsets of the prefixes and compare against wait-k and target-suffix deletion. We also investigate the effect of fine-tuning the target-token generation confidence. Our experiments show that stable prefixes improve the quality-latency tradeoff when translating from English into German, Japanese, and Chinese across a range of test sets.
Anthology ID:
2026.wmt-main.15
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:
254–268
Language:
URL:
https://aclanthology.org/2026.wmt-main.15/
DOI:
Bibkey:
Cite (ACL):
Hieu Hoang, Amittai Axelrod, and Matt Post. 2026. Dynamic Lagging using Stable-Prefix Training for Simultaneous Translation. In Proceedings of the Eleventh Conference on Machine Translation, pages 254–268, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
Dynamic Lagging using Stable-Prefix Training for Simultaneous Translation (Hoang et al., WMT 2026)
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PDF:
https://aclanthology.org/2026.wmt-main.15.pdf