This study investigates the relationship between the linguistic background/s of South African Grade 6 learners and an instrument measuring Core Academic Language Skills (CALS). The learners’ (n = 89) results were divided into two groups based on their most commonly spoken language (either English, L1 or a language other than English, L2). All learners in the study (and indeed the overwhelming majority of South African learners) attend school in an English language immersion setting, where English is the sole language of learning and teaching. The groups were assessed on the CALS-I-ZA, an instrument developed to measure CALS that has been validated in the South African context. This study found that in both the L1 and L2 groups, the CALS-I-ZA showed a strong association with both the provincial Maths Common Examination (r = 0.642) and the provincial Natural Sciences and Technology Examination (r = 0.650). Surprisingly, the home language variable either does not correlate or correlates only very weakly with the other variables, suggesting that L1 or L2 status alone is a weak or increasingly irrelevant predictor of academic success. This research concludes that the strong association between CALS and schooling results remains robust, regardless of L1 or L2 status, and thus, there is compelling evidence to begin deploying the construct as an instructional tool in South African classrooms.