Cory Holland

Madison Van Doren, Casey Ford, Jennifer Barajas et al.

We present a large-scale human evaluation benchmark for assessing cultural localisation in machine translation produced by state-of-the-art multilingual large language models (LLMs). Existing MT benchmarks emphasise token-level and grammatical accuracy, but of ten overlook pragmatic and culturally grounded competencies required for real-world localisation. Building on a pilot study of 87 translations across 20 languages, we evaluate 7 multilingual LLMs across 15 target languages with 5 native-speaker raters per language. Raters scored both full-text translations and segment-level instances of culturally nuanced language (idioms, puns, holidays, and culturally embedded concepts) on an ordinal 0-3 quality scale; segment ratings additionally included an NA option for untranslated segments. Across full-text evaluations, mean overall quality is modest (1.68/3): GPT-5 (2.10/3), Claude Sonnet 3.7 (1.97/3), and Mistral Medium 3.1 (1.84/3) form the strongest tier with fewer catastrophic failures. Segment-level results show sharp category effects: holidays (2.20/3) and cultural concepts (2.19/3) translate substantially better than idioms (1.65/3) and puns (1.45/3), and idioms are most likely to be left untranslated. These findings demonstrate a persistent gap between grammatical adequacy and cultural resonance. To our knowledge, this is the first multilingual, human-annotated benchmark focused explicitly on cultural nuance in translation and localisation, highlighting the need for culturally informed training data, improved cross-lingual pragmatics, and evaluation paradigms that better reflect real-world communicative competence.

Madison Van Doren, Cory Holland

This pilot study explores the localisation capabilities of state-of-the-art multilingual AI models when translating figurative language, such as idioms and puns, from English into a diverse range of global languages. It expands on existing LLM translation research and industry benchmarks, which emphasise grammatical accuracy and token-level correctness, by focusing on cultural appropriateness and overall localisation quality - critical factors for real-world applications like marketing and e-commerce. To investigate these challenges, this project evaluated a sample of 87 LLM-generated translations of e-commerce marketing emails across 24 regional dialects of 20 languages. Human reviewers fluent in each target language provided quantitative ratings and qualitative feedback on faithfulness to the original's tone, meaning, and intended audience. Findings suggest that, while leading models generally produce grammatically correct translations, culturally nuanced language remains a clear area for improvement, often requiring substantial human refinement. Notably, even high-resource global languages, despite topping industry benchmark leaderboards, frequently mistranslated figurative expressions and wordplay. This work challenges the assumption that data volume is the most reliable predictor of machine translation quality and introduces cultural appropriateness as a key determinant of multilingual LLM performance - an area currently underexplored in existing academic and industry benchmarks. As a proof of concept, this pilot highlights limitations of current multilingual AI systems for real-world localisation use cases. Results of this pilot support the opportunity for expanded research at greater scale to deliver generalisable insights and inform deployment of reliable machine translation workflows in culturally diverse contexts.