When working an RFQ in Quoting, the currency initially uses the default currency of the account. You can modify the currency for specific line items on the Responses tab of your quote.
To modify a line item's currency, click on its cell within that column to open its Assign Currency modal. Then use the Select Currency dropdown to choose the desired currency and click on Save to apply.
Once the currency is changed in the Responses tab, the same currency will appear in the supplier portal of the RFQ submitted to the suppliers. Suppliers can still respond in other currencies.