well gettext doesnt have a way to do reverse lookups, but the gettext database format is pretty trivial so you should be able to write your own. You could use the source code of the msgunfmt utility as a starting point. You can fin the .mo files in resources/localization/locales.zip in a calibre install folder. And this is the file spec:
https://www.gnu.org/software/gettext.../MO-Files.html