Title: Do LLMs Perform Multilingual Multi-step Reasoning?
Primary area: foundation or frontier models, including LLMs
Keywords: Multilingual

Abstract: Ideally, large language models (LLMs) should be able to exploit information sources from all available languages to achieve strong performance for diverse tasks, including reasoning. However, most evaluations of multilingual reasoning focus on symbolic domains, e.g., mathematics and coding, and it remains unclear how effective LLMs handle multilingual reasoning in linguistic tasks. In this paper, we introduce a controlled multilingual two-hop question answering setting, where answering a question requires two reasoning steps across two documents in different languages: the first-hop document provides bridging information, and the second-hop document links it to the final answer. Despite the equal importance of both hops, we find that the performance of a strong multilingual LLM (i.e., Gemma-3) is substantially affected by language variation in the second-hop documents more than in the first-hop. To analyze each hop's reasoning process, we evaluate the decomposed sub-questions of a two-hop question. Surprisingly, the model often fails on the first sub-question for inferring bridging entities, yet still answers the overall two-hop question correctly. Our implicit context attribution analysis shows that the model still attends to bridging documents for correct answer generation, despite struggling to interpret them. This shows that the LLM's multilingual multi-hop reasoning does not follow a faithful step-by-step decomposition for sub-question answering. We also find that the absence of reasoning decomposition leads to about 18% composition failures, where both sub-questions are answered correctly, while failing to answer the two-hop question. To mitigate this, we propose a three-stage SubQ prompting method to guide the multi-step reasoning with sub-questions, which boosts accuracy from 10.1% to 66.5%. Overall, our findings shed light on the multilingual multi-step reasoning mechanism and the potential of explicit reasoning decomposition for future tasks.