This research delves into the intricate relationship between language and mathematics education, particularly within the context of mother-tongue-based instruction. It addresses the challenge of reconciling the language of instruction, the learners' mother tongue, and the language of mathematical concepts, emphasizing the need for synchronization to enhance the teaching and learning process. Drawing from international experiences and the Philippine educational landscape, which transitioned to the K-12 curriculum, this study investigates the role of code-switching in Grade-One mathematics education. By conducting a pretest-posttest (...) group design at an Elementary School in Bohol, Philippines, the study evaluates the effects of code-switching within a mother-tongue-based instruction framework. Findings demonstrate that code-switching improves mathematics performance, with the experimental group excelling in advanced and proficient levels post-implementation. Notably, the control and experimental groups exhibited similar pretest scores, indicating their comparability. This research concludes that code-switching is an effective strategy for bridging the linguistic gaps between the mother tongue and mathematical concepts, thereby enhancing the meaningfulness and comprehensibility of mathematics. In light of these findings, the study recommends the adoption of code-switching in teaching mathematics, especially for students with lower academic achievement. Additionally, it encourages future research to explore the applicability of code-switching in other subjects and higher grade levels. This study advocates for an inclusive and adaptive approach to mathematics instruction that recognizes the complex interplay of languages and aims to empower students to understand, participate actively, and develop a profound love for the subject. Based on the empirical findings of this study, it is strongly recommended that the integration of code-switching as a pedagogical strategy in mathematics instruction be formally endorsed within the school setting. This endorsement serves the paramount goal of enhancing the mathematical proficiency and overall academic performance of the students. -/- . (shrink)