it took me a while to notice that! Did Hong Kong merge with Metropolis or something?

Probably becase of MK is techincally based on Chinese cultural things. Merging into each other. Seeing how Metropolis is in Illinois, (And other places) I doubt that this sign has anything to do with a real country and merging with MK stuff... though a very small detail doesn't make it really interesting. Although this thread's topic is.

CHEERS!

In the comics, Metropolis is on the East Coast, bordering the Atlantic Ocean. Sometimes writers say it's in upstate New York, but most often, it's in Delaware.

Along the same lines...
Gotham is sometimes in NY, south of Metropolis, but it's usually in New Jersey.
The Flash protects the twin cities of Keystone and Central City, Kansas.
Green Lantern Hal Jordan is from Coast City, Cali., near the real life Edwards Air Force Base and Los Angeles.
And I believe Wonder Woman lives in Washington DC, but the Amazons' Paradise Island is a more likely candidate for an arena.

however there is an american flag in the superman render on the right hand side

chinatown section of Metropolis maybe?

i dont think around where the daily planet is, but who knows....

Probably due to the merge, Gotham and Metropolis have been hinted that they're the "NYC" of DC world quite a few times.