To be clear, if you are a contractor (self-employed) it doesn't matter where your clients are. You are operating a business in the United States (even if it's just a one-person business). Your tax return is the same, whether the clients are in the United States or a foreign country.
If you are being paid in foreign currency, you have to convert all foreign currency amounts to U.S. dollars. All amounts that you enter on your tax return must be in U.S. dollars.
