Internationalization is already complex enough I wouldn’t overload it with even more possible axis to maintain permutations for. I’d rather look into more purpose built options for the features you think of rather than (ab)using gettext, which is just “bare strings”.
E.g. take a look at Message Format (v1 or v2) and tell me that’s not a lot to work around the constraint that you deal only with strings in internationalization.
