Palawan, Philippines, is characterized by deep cultural heterogeneity and multilingual classrooms, making it a critical context for examining culturally responsive and localized approaches to mathematics education. Anchored on the principles of Culturally Responsive Pedagogy, Ethnomathematics, and Social Constructivism, this study investigated the opportunities and challenges of contextualizing and localizing mathematics instruction and examined its implications for teaching practices, student learning, and curriculum development in this culturally complex frontier region. Employing a sequential explanatory mixed-methods design, quantitative data were first collected through structured surveys from 100 secondary mathematics teachers and 300 students to identify general trends in perceptions of localized instruction. These findings then guided the qualitative phase, which involved semi-structured interviews and focus group discussions to explain and deepen understanding of the quantitative results. Findings revealed that contextualized and localized mathematics instruction significantly enhanced student engagement, motivation, conceptual understanding, collaboration, and cultural identity, as lessons grounded in community experiences and familiar practices made abstract concepts more meaningful and accessible. However, several systemic challenges were also identified, including limited availability of high-quality localized instructional materials, increased teacher preparation demands, linguistic diversity within single classrooms, misalignment with standardized curriculum and assessment systems, and inconsistent institutional support. This study contributes new empirical evidence on the tension between linguistic plurality and standardized educational structures within Indigenous–migrant learning environments, a dimension that remains underexamined in Southeast Asian mathematics education research. Sustainable implementation requires targeted teacher training for multi-ethnic classrooms, alignment of localized instruction with curriculum and assessment systems, adequate instructional resources, and stronger institutional and community support.