No such thing as a monolingual human. Any language can be broken down to subsets that are associated with different ways of thinking. Another thing is globalization and culture export.

I know plenty of bilingual people that have a very limited understanding of the world, and conversely monolinguists that have a very broad view.

One could even say assuming someone's level of worldly understanding based on how many languages they speak shows a fairly limited world view.

As I speaker of five languages, all but one fluently: why does my understanding of the world magically increase when I learn a new noun so say "sparrow" in the fifth that I'm learning?

Is it linear (25% more understanding for the fifth) or asymptotically? Does it increase across all domains equally (geology, poetry, ethics) or asymmetrically?

Seriously, explain it to me?