Language policy plays a critical role in shaping socio-economic outcomes and human capital development across nations. This chapter investigates the impact of official language choices on education, economic opportunities, and societal cohesion. The concept of Average Distance from Official Language (ADOL) is introduced as a quantitative measure of linguistic distance, which significantly influences learning costs, literacy, and productivity. ADOL is constructed by calculating the linguistic distance between the mother tongue of the ethnic groups and the official language of the country. Cross-country analyses reveal that lower ADOL scores result in improved cognitive skills, life expectancy, and GDP per capita, while higher scores exacerbate disparities. Case studies from Ethiopia, Cameroon, and South Africa highlight the advantages of mother-tongue instruction in primary education, including improved literacy rates, higher educational attainment, and better labor market outcomes. Theoretical frameworks highlight the role of written traditions and linguistic diversity in determining language policy choices. Nations with strong written traditions are more likely to prioritize indigenous languages, while those with high linguistic diversity tend to retain colonial languages. These findings underscore the importance of aligning language policies with linguistic realities to reduce educational barriers and promote equitable development.