Improving LLMs for Machine Translation Using Synthetic Preference Data

Dario Vajda; Domen Vreš; Marko Robnik-Šikonja

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arXiv · 2508.14951

Improving LLMs for Machine Translation Using Synthetic Preference Data

Dario Vajda·Domen Vreš·Marko Robnik-Šikonja

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Abstract

Large language models have emerged as effective machine translation systems. In this paper, we explore how a general instruction-tuned large language model can be improved for machine translation using relatively few easily produced data resources. Using Slovene as a use case, we improve the GaMS-9B-Instruct model using Direct Preference Optimization (DPO) training on a programmatically curated and enhanced subset of a public dataset. As DPO requires pairs of quality-ranked instances, we generated its training dataset by translating English Wikipedia articles using two LLMs, GaMS-9B-Instruct and EuroLLM-9B-Instruct. We ranked the resulting translations based on heuristics coupled with automatic evaluation metrics such as COMET. The evaluation shows that our fine-tuned model outperforms both models involved in the dataset generation. In comparison to the baseline models, the fine-tuned model achieved a COMET score gain of around 0.04 and 0.02, respectively, on translating Wikipedia articles. It also more consistently avoids language and formatting errors.

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Dario Vajda, Domen Vreš, Marko Robnik-Šikonja. 2025-08-20. Improving LLMs for Machine Translation Using Synthetic Preference Data. https://arxiv.org/abs/2508.14951

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