I think diversity is hugely important for avoiding groupthink and opening minds to new possibilities. I work in International Development, and the vast majority of failures and missteps in my field are due to people designing programs that make perfect sense to a white first-world male, and end up having some pretty huge problems in actual implementation. I can’t imagine my education being effective without the huge number of international students and diversity of experience that we all brought in.
Even something seemingly as objective as medicine is hugely influenced by culture and diversity. Vaccines developed in US labs, for example, often function completely differently in other places. The rotavirus vaccine, for example, is only half as effective in Africa as it is in the West, and nobody is quite sure why. Culture affects what side effects people are willing to tolerate, what treatment schedules they are going to stick to, and even what it means to be “sick” and “well.” If a doctor does not come to understand other cultures, they will not be as good of a doctor as they could be.
As for language- there are a lot of multilingual countries (pretty much all of Africa, India, China and much of the rest of Asia, etc.) and I can’t think of a single one that doesn’t basically have a de facto first language for trade, business and other interactions across a large region. Even in areas where this is not organized officially, these languages emerge naturally. 66% of the world’s children are raised bilingual. The idea of a completely fragmented society where nobody can talk to each other is not a reality and never has been one.
Linguistic diversity has some advantages. For one, knowledge is often stored in languages. We know for a fact that certain Amazonian languages have many more words for local plant and animal species than we do, showing that they have knowledge of their environment that hasn’t yet been “discovered” by western science. As these languages die out, we are losing that knowledge and will have to rediscover it from scratch.