Fix resolveBrowserLocale tests

[FernandoKGA]

Noticed a bug at resolveBrowserLocale tests during #7170 investigation to implement a new feature (#7193).

Problem:
As navigator.language is a read-only property (see MDN page about it), it wasn't being affected by the window.navigator = {}. You can see that at should return default locale if there's no available locale in browser test, it is an invalid test.

Solution:
Implement correctly using jest.spyOn function, see this answer: https://stackoverflow.com/a/52870312

Should this fix enter before #7193 @WiXSL ?

[WiXSL]

Thank you for taking the time to work on this.

[djhi]

Thanks!