Then everyone needs to be able to communicate with each other. Well almost everyone anyway.
Humans mainly interact in three ways: cooperation, trade, or force... and variations thereof.
In only one of these is communication less vitally important... guess which one.
It is difficult enough to engage in trade or cooperation with those whose culture, customs and norms differ dramatically from what you're used to... add in an inability to communicate clearly and the difficulties became almost insurmountable.
Diplomats have first-class translators to help them understand the people they are negotiating with, including parsing nuances in the language.
You don't have that. When you find yourself in a potential conflict or misunderstanding with people who don't speak any language you know, resolving that conflict is going to be extremely difficult.... let alone feeling comradely or brotherly towards those whose words and intentions are unknowable to you.
Forcing everyone to learn all of the world's languages is not the answer. There are at least 6,500 languages currently spoken on Earth. Even if we limit it to the relatively common ones, there are 23 languages spoken by about half the world's population.
Most people have enough trouble learning ONE new language, maybe two. There's a reason people who speak 5 or more languages are rare. It is difficult for most people to learn new languages.
The answer seems obvious: everyone learn one language, the SAME language. It can be a second language, in addition to your native one, but it needs to be the one everyone else learns... then communication is far easier, trade and cooperation become much more feasible and violence less likely.
At present, English is the most common second language in the world. That's a good start, but English or something needs to be taught universally as a 2nd language if we expect to ever have even the slightest chance at this "peace and brotherhood across the planet" idea.
The current number of English speakers, both native and second language, is estimated to approach 1 Billion already. 55 nations officially teach English as a second language; English speakers can be found individually in almost any nation in some numbers. Seems like an obvious choice.
Or if you don't like English fine offer an alternative, but SOME single language needs to become the Common Tongue of Earth if we're ever going to get anywhere together as Species Humanity.