Nothing is 100% gain or loss.
But I believe there is nothing in our current situation that is worth more than having a united humanity, which can only happen if well all can understand each others.
> There are some things that are simply untranslatable to English - these words/concepts are closely tied to the way we live
It's ok, you can use the native word in an english sentence. English does that all the time: "je ne sais quoi" (french), "taxi" (turkish), "pizza" (italien), etc.
> Now if everyone in my town starts speaking only English suddenly, it would definitely affect the way they think[1], function, and would inevitable change the culture.
Yes. Change is inevitable anyway. But we can choose to change in one direction. I think the direction of uniting the human specie is the best choice.
> preserving a language might help preserve a culture.
I'll exchange my own entire french cultural heritage in a blink if suddently I could make the entire world speak one common language.
But it doesn't have to be.
A culture is not this immutable things we think it is. Most thing we call ancestral are really just the result of a continuous transformation process that have the same flavor, and so that we identify as stable. Even centuries old religions have been changing all the time.
So culture will change as well. It will adapt, and keep what is meant to be kept, then drop the rest.
We will lose some valuable things in the process, but to gain much more.