You would have to move a piece of logic code from python to JS (detect and associate plural rules for one given key), but I don't see any major thing that would prevent reading directly .properties. And tbh, when we implemented l10n, I wasn't for using JSON intermediate format... I wish we could have used .properties (or even better, yaml) in the xpi. But myk convinced me to use JSON intermediate format in order to ease integrating with l10n website that was never released :(