Add locale to EnvironmentManipulationPanel

[bryanoltman]

A user should be able to specify a list of locales supported by their app. These locales (if specified) should appear as options in a popup menu in the EnvironmentManipulationPanel.

[Toavina23]

I have implemented this feature into my fork, and I would like to know when will I be able to fill a PR for it knowing that contributions are not accepted yet

[bryanoltman]

This repository is being migrated to https://github.com/google/flutter-stager and you should feel free to open a PR there – it just may take a bit to get reviewed

[Toavina23]

Alright, thanks I will do just that