zAkeno,
This is the suggested route for changing your country flag. I hope this helps you.
We can change this for you, but only if you are actively living in the country that you wish to change to.
We determine your country as shown on your profile page from the area in which you first registered your account from.
If you create an account while on holiday in the United States, your account will show as belonging to the US - you'll even appear on the country rankings for the US too.
The support team can happily change your account's country upon request, but only if you are actively living within that country at the time.
To confirm this, the team will ask for proof of residency - be it the appropriate passport, drivers license for your country, or some other form of ID. This must be displayed in a picture next to your monitor with the osu! client open and logged in to your account.
We will never display this information to anyone else, and we encourage you to blank out any personal information (that will not otherwise make it impossible for us to determine what country the document is for!) if you are concerned about your privacy.