FR: Provide an option to set languge for activation links

[ghost]

• Operating System version: Any
• Firebase SDK version: Any
• Library version: 7.0.1
• Firebase Product: auth

Currently it seems like there is no way to set the languge in this metod: com.google.firebase.auth.AbstractFirebaseAuth#generateEmailVerificationLink or in its Async coutnerpart.

The returned url always ends with &lang=en Firebase Auth supports other languages if the end of the url is changed to another language the users land on a page on that given language. It seems like that the Javascript library has this option already: https://firebase.google.com/docs/reference/js/firebase.auth.Auth#languagecode

It would be great to be able to set this directly from the library.