Transparency in a flag is new to me. It's surprisingly effective here. Could this be done with just cloth without synthetics by using thinner threads or thread count?
The website mentions silk organza at https://1worldflag.com/flag/. That's where my mind went initially, some kind of sheer silk fabric. I don't know how durable it would be in the elements though.
But the transparency :( It makes the whole flag low contrast, an undesirable quality in flags.