Cómo cambiar un texto de botón Kivy en Kivy Lang desde el archivo Python

from kivy.app import App
from kivy.lang.builder import Builder
from kivy.properties import StringProperty
from kivy.uix.boxlayout import BoxLayout


class Container(BoxLayout):
    message = StringProperty()

    def retranslate(self, language):
        texts = {"en": "Hello World", "fr": "Salut monde"}
        self.message = texts.get(language, "")


Builder.load_string(
    """
<Container>:
    orientation: 'vertical'
    Button:
        text: root.message
    Button:
        text: "Eng"
        on_press: root.retranslate("en")
    Button:
        text: "Fra"
        on_press: root.retranslate("fr")
"""
)


class MyApp(App):
    def build(self):
        w = Container()
        w.retranslate("en")
        return w


if __name__ == "__main__":
    MyApp().run()

Depressed Donkey