Como alterar um texto de botão Kivy em Kivy Lang do arquivo 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