Can I add something to this questions?

Why are there so many crosses (Scandinavia, switzerland, Greece, finland, iceland, britain) and tri-colours (France, Germany, Russia, Hungary, Italy etc) in european flags?

Not a historian, but I love vexillology!

The crescent moon is a symbol of Islam in the same way a cross is a symbol of Christianity, so many Asian countries display the symbol on their flag (fun fact: Turkey is the first country to have adorned it's flag with a crescent moon).

Circles really aren't that common on Asian flags...I believe there are only five countries that use one:

• Japan- "Source of sun"
• Bangladesh - a red sun to symbolize blood spilled in the battle against pakistan
• North Korea - white circle to symbolize the universe
• South Korea - a yin yang/ Taegukgi to symbolize infinity
• Laos - circle to symbolize a white moon rising over the Mekong River

A much more popular symbol on Asian flags would be a star, which also represents Islam in most cases.

P.S. Flags Of The World by Sylvie Bednar is a great book for learning more if you're interested.