Historically, African languages have been in a disadvantaged position and issues related to dialect are not fully understood and acknowledged. In this study, we examined the influence of dialects and code-switching on the literacy and numeracy achievements of isiXhosa Grade 1 learners with Multilevel Modeling (MLM), when controlling for other factors, such as between-school variance, socio-economic status, and sex. The project used stratified random sampling to select Afrikaans, English, and isiXhosa schools in three districts in the Western Cape. A total of 2 497 learners were assessed, of whom 768 were in isiXhosa schools. Dialects and codeswitching had a significantly negative influence on both literacy and numeracy scores of isiXhosa learners. The findings highlight the importance of investigating dialectic issues in isiXhosa and the implications that dialects have for decolonisation. African language decolonisation requires the development of classroom resources for these languages, increased African educational research, and finding ways to address the complexities of dialects in pedagogical frameworks.