Is there no multilingual support for the payment pop-up? I don’t find any way to edit it…
No customization is available at the moment for Wix Pay API checkout window as far as I know, since it is not a default Store checkout.
However, if you decide to use default Wix Stores - they have more language options available: https://support.wix.com/en/article/languages-supported-in-wix-stores
In this case you can contact Wix Support for advice on customizing your checkout:
https://support.wix.com/en/article/contacting-wix-support