SwiLTra-Bench: The Swiss Legal Translation Benchmark
Authors: name: Joel Niklaus, name: Jakob Merane, name: Luka Nenadic, name: Sina Ahmadi, name: Yingqiang Gao, name: Cyrill A. H. Chevalley, name: Claude Humbel, name: Christophe Gösken, name: Lorenzo Tanzi, name: Thomas Lüthi, name: Stefan Palombo, name: Spencer Poff, name: Boling Yang, name: Nan Wu, name: Matthew Guillod, name: Robin Mamié, name: Daniel Brunner, name: Julio Pereyra, name: Niko Grupen

In Switzerland legal translation is uniquely important due to the country's
four official languages and requirements for multilingual legal documentation.
However, this process traditionally relies on professionals who must be both
legal experts and skilled translators -- creating bottlenecks and impacting
effective access to justice. To address this challenge, we introduce
SwiLTra-Bench, a comprehensive multilingual benchmark of over 180K aligned
Swiss legal translation pairs comprising laws, headnotes, and press releases
across all Swiss languages along with English, designed to evaluate LLM-based
translation systems. Our systematic evaluation reveals that frontier models
achieve superior translation performance across all document types, while
specialized translation systems excel specifically in laws but under-perform in
headnotes. Through rigorous testing and human expert validation, we demonstrate
that while fine-tuning open SLMs significantly improves their translation
quality, they still lag behind the best zero-shot prompted frontier models such
as Claude-3.5-Sonnet. Additionally, we present SwiLTra-Judge, a specialized LLM
evaluation system that aligns best with human expert assessments.