This has been my experience with picking up a second language. There are concepts that are expressed in a single Japanese word that seem like they would need a small book to explain adequately in English. Maybe there are places where the language and culture aren't so intertwined, but Japan at least isn't one of them. So as much as I think the grand flattening of the world is great in some ways, I would be a little sad if we ended up with a global monoculture catalyzed by the coalescing of languages. Other cultures are just fascinating to explore.

As a native French speaker, I agree with you. For technical discussions such as programming or engineering it can be argued that using one language (nowadays, obviously English) encourages cooperation and progress. However, in other domains, languages carry much more than a practical means of transmitting information, they bear a whole culture with them. Two examples which I hear and read a lot in the English speaking world but which are either impossible to translate in French or at least to utter without ridicule: "successful" (as in a person), and "evil" (as in, a dictator).

In my opinion (as someone who works in NLP & computational linguistics), preserving more data on how languages can work is the most important bit. Languages do influence cognition a little bit, but not as much as most people think. Nevertheless, most people think that they do, and if you only know a few languages, its easy to get stuck thinking that, oh, this is how all languages must work, and this is inherently how people think.

This is a problem in contemporary linguistics- the "big" languages are really heavily studied and theories are developed to describe them accurately, nevermind that they may completely fail for, say, Xhosa or Straits Salish. The bias has been decreasing over the last few decades, but getting a really good idea of the real fundamental underpinnings of human language depends on all those minority languages not disappearing, undocumented, before we're done.

Of course, there are other reasons which are important to people working on other fields- languages are strongly tied to culture, and when a language dies, so does a lot of other cultural knowledge and practice that may be useful to ethnologists, psychologists, philosophers, etc.

It's my understanding that the language you speaks has only a small effect on your outlook. The gist behind the Sapir Whorf hypothesis was that the number of concepts you could grasp was limited by the language you spoke. But I don't think most linguists seriously believe this anymore. Research has shown language to influence our outlooks in limited ways, and even those findings are suspect, since some of the differences can be explained by cultural differences.

A good overview: https://en.wikipedia.org/wiki/Linguistic_relativity