Use Content negotiation
It is not recommended to use this:
GET /project/fubar/fishes.en-us.json
In REST terms, this is a different Resource than those at
GET /project/fubar/fishes
But as far as I understand your question, both are the same Resource, only in different Representations.
To get a Resource in a specific Representation, use Content negotiation.
GET /project/fubar/fishers
Accept: application/json
Accept-Language: en-us
Where are the items?
I don't see any items in your URLs, only what you call 'folders'. In REST terminology, these are called Collection Resources. You add an item to a Collection Resource by POST
ing to it. YOu delete an item by DELETE
ing it. You change parts of an item by POST
ing the changed parts to it. You update a complete item by PUT
ing to it.
I recommend to identify the Collection Resources and the items in your system.