To be able to Publish your app to Flutter, you will need to disable the multilingual content in your app.
- Open your app
- Navigate to Settings > Advanced
- Make sure that you have selected the desired default language. This will be the language that the app content will be available in your app.
- Under "Languages" deselect the languages that you will no longer be supporting in the app.
Once the languages have been removed, you can go to the Publish page and continue publishing for Flutter.