Sets the fallback locales.

        @type locales: list[str]
        @param locales: The falback locales

        @raises: ValueError: If a locale contains invalid characters

def set_fallback_locales(self, locales):
        """
        Sets the fallback locales.

        @type locales: list[str]
        @param locales: The falback locales

        @raises: ValueError: If a locale contains invalid characters
        """
        # needed as the fallback locales are linked to the already loaded
        # catalogues
        self.catalogues = {}

        for locale in locales:
            self._assert_valid_locale(locale)

        self.fallback_locales = locales