I didn't even see that at first, because France seemed such an obvious answer. But now that you mention it, the right side of the vertical blue stripe on the flag of Norway gives the same pattern.
Well noticed! It's very close up though! (I think the shades are a little different too, but that would be the most nitpicky thing even for this website!)
I am in awe of people who can remember all the world capitals and all the world flags. I am a dismal failure at both, but I am very slowly making progress.
I tried to specifically look for a version with a red background and the closest one I found (removed the white, so it was half red, half blue) was from a Reddit post titled "My Redesign of the Haiti Flag".
Singapore and Georgia almost had me, and l had to bargain Salahaddine's Eagle between Syria and Egypt. But at the end of the day it's full marks IV for me!
I'd hate to be a nitpicker but you should accept Poland for the sorry excuse of "Japan" because when the Polish flag is upside down, curved a little bit, and has a different hue of red it looks like that so...
In 1999, Japan redesigned its flag
Otherwise that's like saying France has the tricolor so you should accept Belgium for that answer.
I tried to specifically look for a version with a red background and the closest one I found (removed the white, so it was half red, half blue) was from a Reddit post titled "My Redesign of the Haiti Flag".