Evaluating the Effect of Prompt Language on LLM-based Translation: Evidence from Spanish<>Italian Translation

Antonella Bove, Paola Di Cataldo, Davide Maestroni


Abstract
The integration of large language models (LLMs) into translation practice has substantially reshaped translation workflows (Kornacki and Pietrzak, 2025). Since translation quality depends partly on how these models are prompted, prompt design deserves closer attention as a key stage of the LLM-augmented translation process. This study investigates Spanish<>Italian translation with GPT 5.1 in the advertising and biomedical domains. It examines whether prompt language affects the quality of translations generated with the GPT 5.1 model, and specifically whether prompts written in the target language outperform prompts written in English, the language most prevalent in the model’s training data (Armengol-Estapé et al., 2022). Three prompt templates, varying in complexity and informational content, were tested. The translations were first screened for textual similarity, and only the translations generated from the template that produced the greatest variation across outputs were subsequently selected for human evaluation. Human judgments were collected through a pairwise-comparison task. The findings indicate that prompts written in the target language tend to yield higher-quality translations compared to prompts written in English.
Anthology ID:
2026.eamt-1.51
Volume:
Proceedings of the 26th Annual Conference of the European Association for Machine Translation (Volume 1)
Month:
June
Year:
2026
Address:
Tilburg, The Netherlands
Editors:
Dimitar Shterionov, Eva Vanmassenhove, Mirella De Sisto, Fred Blain, Javad Pourmostafa Roshan Sharami, Lisa Lepp, Chiara Manna, Argentina Anna Rescigno, Alina Karakanta, Ayla Rigouts Terryn, Manuel Lardelli, Natalia Resende, Elena Murgolo, Janiça Hackenbuchner, Anna Zaretskaya, Miquel Esplà-Gomis, Thierry Etchegoyhen, Dagmar Gromann, Rachel Bawden, Barry Haddow, Sara Szoc, Mikel Forcada, Helena Moniz
Venue:
EAMT
SIG:
Publisher:
European Association for Machine Translation
Note:
Pages:
800–812
Language:
URL:
https://aclanthology.org/2026.eamt-1.51/
DOI:
Bibkey:
Cite (ACL):
Antonella Bove, Paola Di Cataldo, and Davide Maestroni. 2026. Evaluating the Effect of Prompt Language on LLM-based Translation: Evidence from Spanish<>Italian Translation. In Proceedings of the 26th Annual Conference of the European Association for Machine Translation (Volume 1), pages 800–812, Tilburg, The Netherlands. European Association for Machine Translation.
Cite (Informal):
Evaluating the Effect of Prompt Language on LLM-based Translation: Evidence from Spanish<>Italian Translation (Bove et al., EAMT 2026)
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PDF:
https://aclanthology.org/2026.eamt-1.51.pdf