Well, Geany could provide an `appdata.xml` but it would be very bland, it could have no distro specific information in it, like "GTK3" in the example you gave. So distros may not use it anyway.
And it would have to be integrated with the internationalisation system properly (`appdata.xml.in` maybe?).
If somebody made a pull request that did that, we can query our packagers to see if they think its worth it.