Large Language Models (LLMs) excel at mathematical reasoning in English, but their performance in low-resource languages remains underexplored. This gap is particularly critical in the Indonesian context, where equitable access to AI systems depends on robust multilingual reasoning across diverse local languages.We introduce MATH-IDN, a multilingual benchmark for mathematical problem solving in Indonesian, Javanese, Sundanese, and Buginese, with English as a reference, following the MATH dataset. We evaluate multiple open-source LLMs, including math-specialized, Southeast-Asian-adapted, and general-purpose models, under a zero-shot chain-of-thought setting. Results show that MATH-IDN presents a challenging and discriminative benchmark, revealing substantial performance gaps in low-resource languages, particularly Buginese, and highlighting key limitations in current multilingual reasoning capabilities. Our data and code are available at https://github.com/aialt/MATH-IND.