Good day, Daniel_Benes.
The online payment link for invoices is only available for customers in the United States. However, you'll want to check for a third-party application that can let your customers pay you online. Check the QuickBooks App Center.
Here's an article to gain more insights on how multi-currency works in QBO. This way, you're able to learn what happens before and after turning it on: How Multi-currency works in QBO?
Please let me know how it goes, I'd love to help you more.