Budget Alignment: Making Models Reason in the User's Language

LLMs often reason internally in English even for non-English queries, limiting faithfulness and weakening human oversight in multilingual settings. We study budget alignment: lightweight methods to align a model’s reasoning language with the user’s language under modest data and compute. Using a 7B model, we evaluate multilingual SFT, RL for accuracy recovery, and model merging. Across Japanese, French, and Spanish tasks, these approaches markedly increase language-consistent reasoning while preserving strong accuracy, showing that faithful and interpretable multilingual reasoning can be achieved with low-cost alignment.