Add localization support

[cygnet3]

Add support for different languages using 'flutter_localizations'.

[yashrajd]

This could benefit from a choose language drop-down during app onboarding... or just shove it in settings behind a list-item.

[cygnet3]

I think it's best to ask the user immediately when they first open the app, so that would be during wallet creation.

Perhaps flutter provides some way to see what language the phone itself is set in? Then we don't need to ask the users, we can just use that.