With 872 languages, West Africa is the second largest hotspot of language diversity in the world, surpassed only by New Guinea (1,065 languages). Together, these regions comprise more than a quarter of the world’s languages. We analyze individual-level parameters of family language use, socioeconomic status, life skills, and language traits as factors influencing language proficiency and ethnobiological knowledge for 831 students in Cameroon who speak 65 languages (24% of the country’s total) and compare them with data from 6,190 students speaking 392 languages in Papua New Guinea (PNG). We show that only 54% of Cameroonian students are fluent in indigenous languages compared to 92% of their parents. Urbanization, traditional skills, and language use in the home predicted students’ language proficiency, which in turn was strongly correlated with their ethnobiological knowledge of birds and plants. Drivers of language proficiency were nearly identical in Cameroon and PNG, but PNG students had better ethnobiological knowledge than Cameroon students across all levels of language proficiency. The rapid decline in language proficiency and ethnobiological knowledge in both language hotspots is likely to continue in the future, particularly due to increasing urbanization and globalization in both countries, which in PNG is also leading to an increasing proportion of linguistically mixed families.