Wow!... I admit I didn't look at that file as thoroughly as I should have, before assuming it wouldn't work
-- simply because until now I had only seen incomplete or mutually conflicting files concerning locales, languages, codes etc.
But you're totally right - that file seems to contain everything that we need! (And what's more, it's internally consistent!)
So I have just found my new task: generate meaningful properties files for the locales that have been registered for translation.
@eureka: thanks a ton for the pointer and the clarifications.