It should not be hand-made though, but generated from the list of languages and with completion percentages extracted (like it’s done for the manual for instance in: https://docs.gimp.org/).
Reading the top of that page I think the lang tag is for downstream usage, like for software catalogs.
For single packages it looks like Upstream Metadata | AppStream | AppStream 1.0 is enough. (GIMP already has <translation type="gettext">gimp20</translation>, so one would expect it to be fine.
When testing I saw many programs with the AmE banner in gnome-software (Boxes, Builder, ghex, Maps and dozens others), so I will file a bug there when I have thought some more about it. I would expect most packages to have over 50% that I saw was the threshold.
I also got the banner on many different modules that should be translated, interestingly not on GIMP. Might be something that changed with language detection on Flathub, but I’m not sure, so started out with an issue against gnome-software: