I can’t imagine the world you inhabit. It is a joy to learn another language, to be able to read a poem as it was intended to be read, to understand the lyrics of a song, to have the kind of expanded awareness that comes from the other language as it enhances your native language.
And your statement that people don’t really speak other languages well is just untrue - I’ve traveled, worked - and done simultaneous translation - in multi-national situations, and everyone foreign spoke perfect English.
I fully agree.
The nuances and shades that exist in one language don't exist in others. For instance in French there are multiple ways to describe how you do things that puts english in shade, in Polish the language is highly precise, but english is the best in precision business and technical terminology -- goals/objects/targets can mean the same word in Polish for instance.
Also, different languages require you to think differently, use different parts of the brain.