I'm sure common language helped Russia and Ukraine when they went into war.

What you're looking for is single worldwide culture. But I doubt that could happen. E.g. colonies in "New World" soon departed from their parent cultures. Even if someone would establish a single culture, it's set to collapse and split into a myriad of smaller ones.

There're a lot of things that define culture. Looking at US coasts-vs-flyover divider, language is not what keeps people together. Even if they shared a somewhat similar culture, different environments make people go in different ways.

> I really disagree with this idea that the death of a language somehow means we are cut off from the culture that used the language.

Culture is much more than just translating words. Different people care more about different things. That PoV is lost when nobody speaks the language natively. Well, it may exist in a book in a library. But there's no way to experience the culture anymore. So yes, it's dead, but not lost.