It isn't necessary to have only one language for people to communicate. Humans are perfectly capable of learning multiple languages, and and it may even be good for a child's cognitive development to do so.
It's also unlikely that it is possible to have one global language. People acquire language from the people they grow up around. They get it from their parents, friends, neighbors, etc (note that I omitted teachers). Even if everyone had the possibility to communicate with any other person in the world, from birth, they wouldn't, and so dialects would form and eventually become languages.
It's also unlikely that it is possible to have one global language. People acquire language from the people they grow up around. They get it from their parents, friends, neighbors, etc (note that I omitted teachers). Even if everyone had the possibility to communicate with any other person in the world, from birth, they wouldn't, and so dialects would form and eventually become languages.