where $attacker and $defender are separately formatted creature-* strings.

Yep, that's what we do right now 😅

As for that

This doesn't work for languages with more than two plural forms, and it's mixing up terms that should be translated separately. You should instead do something like:

Yes, obviously. That's why I said that it would require allowing different languages define their own terms.
So it could be -creature-one, -creature-few, -creature-other for all number variants specific language needs. Even if round-trip via formatting is probably a better solution.

And I agree that it's rather clunky, that's why I showed second example with one big selector.
And example with cases and genus wouldn't work that way, because only that language can say what gender certain creature has.
Not to mention cases, which are wildly different in different languages.

I'm not sure what's the best way to handle more complex situations, especially if you need gender, plurality and grammatical case all at the same time.

But I still wanted to give examples what seems possible with this MR.
Sorry, if it was more of a noise ^^'