When creating a draft order, you can't directly set the currency via the currency field in the REST API, so `"currency": "czk",` in your code above won't force currency to be CZK. Instead, the currency is determined by the customer's currency or the store's default currency.
Is this draft order for a customer that already has a customer account associated with them? If the draft order doesn't have a specific customer attached to it, Shopify will use the store's default currency for that draft order. However, if the draft order is associated with a specific customer, Shopify will use that customer's default currency for the draft order. You can also manually change the default currency for specific customers in the customers section of the admin.
This is for a customer who, after creating an order. via api will be redirected to checkout and pay for the order there. I Such a customer MUST see the CZK currency where he will choose the form of payment, etc.