Wix actually have a very simple solution to customize the hreflang, as follow:
- Go to the Multilanguage module from “Manage Languages”.
- Click on the “… icon” and select “Set regional format” for the language you want to custom the hreflang tag.
- Select the Region locale you want (afterwards hit publish to be safe).
- Check the hreflang tag on the web page to see if you get the desired result.