add localization delegate and ja_JP button text.

[renoinn]

Hi tom. i try localization this plugin for japanese. please using this code, if looks good to you.

https://developer.apple.com/documentation/signinwithapplejs/incorporating_sign_in_with_apple_into_other_platforms

[NiKoTron]

I guess that localisation dictionary should be available to extension from the outside.