Education for multilingual Physics students in South Africa still has a monolingual bias despite such pedagogy being repeatedly identified as the key factor in students' academic underachievement in the subject. The paper reports on the pivotal role language plays in the comprehension and subsequent academic performance of students in science education. A mixed-methods approach was used to generate responses from a sample of 40 eleventh-grade students on the role of language in the learning and teaching of Physics in a multilingual South African high school. Data was collected from pre- and post-tests as well as interview responses. Participants were randomly assigned to either the control or experimental group using Research Randomizer and those in the experimental group were given intervention in the form of lessons and learning materials in their mother tongue. A paired t-test disclosed a statistically significant difference in the academic performance of the two research groups in the post-test. The present research findings are in line with previous research which affirms that the use of students' mother tongue promotes a deeper understanding of Physics concepts resulting in improved academic performance.