All of humanity speaking one language, and having multiple languages, are not actually mutually exclusive. Diglossia and multilingualism are the norm. There's

All of humanity speaking one language, and having multiple languages, are not actually mutually exclusive. Diglossia and multilingualism are the norm. There's no reason why people can't speak a mother tongue at home and in a local community, and use an international language for math, science, and travel. The question is whether people should be coerced into abandoning their mother tongue.