Adding support for multi-currency accounts

If a connected account supports multiple currencies, customers can now retrieve the balances for each supported currency from the dashboard and the API.

When a payment order is created on a multi-currency account from the dashboard or the API, the credited or debited balance will be the balance of the currency selected for the payment order.

When creating a new payment order from the dashboard, customers can also visualise the balance for each currency of a multi-currency account when selecting the connected account.