Can I change the default language of the banner?

It is not possible to change the default language of the banner, which is English in the Usercentrics for Wix app.
However, you can add any available language via the Banner > Content > Language Settings option..
The language displayed in the banner depends on a combination of factors - if a language other than English is added via the app, users will then be presented with an icon at the top right of the banner, where they can select any of the languages previously added.