URL aliases with default fallback

Hello,

I've got a multilingual website where users can translate nodes.

When users create nodes in some language, an URL alias will be created only for that language. In other cases, /node/%node will be used.

If an URL alias doesn't exist for a particular language, I'd like the system to use a default one.

For instance:
- I create a node in French, the generated alias is: fr.example.com/mon-titre
- If I don't translate it to English (which will be the most likely use case), users on the English site will access it from: en.example.com/node/12
- I'd like users on the English site to access it from: en.example.com/mon-titre

Would anyone know how to do that?

Thanks,
saipas