Help Center wrote:
Your nationality is automatically determined by the game based on where you register your account from geographically speaking, meaning that if you register an account in the US, your account will be automatically assigned as being from the United States, and also placed within the US country rankings.
We can change this upon request, but you must provide proof of residency for the given country that you wish to change to. A picture of any sufficient 100 points of ID (passport, driver's license) representing this alongside a slip of paper with your osu! username written on it is required for us to even consider your request.
Providing false information to us may result in your account being placed under permanent restrictions, or heavily silenced for upwards of a month.
Contactaccounts@ppy.shwith proof or residence for a flag change.