When Models Reason in Your Language: Controlling Thinking Trace Language
  Comes at the Cost of Accuracy
Authors: name: Jirui Qi, name: Shan Chen, name: Zidi Xiong, name: Raquel Fernández, name: Danielle S. Bitterman, name: Arianna Bisazza

Recent Large Reasoning Models (LRMs) with thinking traces have shown strong
performance on English reasoning tasks. However, their ability to think in
other languages is less studied. This capability is as important as answer
accuracy for real world applications because users may find the reasoning trace
useful for oversight only when it is expressed in their own language. We
comprehensively evaluate two leading families of LRMs on our XReasoning
benchmark and find that even the most advanced models often revert to English
or produce fragmented reasoning in other languages, revealing a substantial gap
in multilingual reasoning. Prompt based interventions that force models to
reason in the users language improve readability and oversight but reduce
answer accuracy, exposing an important trade off. We further show that targeted
post training on just 100 examples mitigates this mismatch, though some
accuracy loss remains. Our results highlight the limited multilingual reasoning
capabilities of current LRMs and outline directions for future work. Code and
data are available at https://github.com/Betswish/mCoT-XReasoning.