Your audience can pay their plans in virtually any currency. If the currency is not EUR or USD, the payment service provider (for example, the credit card provider) will convert your plan's price the local currency. A conversion fee might over which Steady has no control might apply.
The project currency determines if plan prices are displayed in EUR or USD. If someone pays for a EUR project with a EUR payment method (e.g. a credit card linked to a EUR bank account), no conversion fees apply. The same is true for payment with a USD payment methods to USD projects.
You must specify the project currency when creating your project. This cannot be changed afterwards.