Hi,
wondering from where you took your configuration and that there is no documentation in the Grav documentation, I was browsing through the code and finally found your use case :-).
To answer your question: you can simultaneously set both options, but not in the way you did it, but rather
languages:
translations:
fallback: true
This sets languages.translations automatically to true.
However if you want to disable the translation option, you will have to delete your custom modifications, i.e. you have to return to
languages:
translations: false
then.
Best regards!