I am facing the same scenario. Mobile app is been developed with Flutter and, as the Rest API tests are implemented using Robot Framework we would like to extend out coverage to mobile UI using the AppiumLibrary available with Robot Framework.
They would need appium library to interact with the mobile application, overall it makes little to no difference what language the mobile app had been developed with.
There’s a dedicated library for flutter applications, it’s currently beta and I haven’t experimented with it.