Latest gettext already ships the following files:
But these only describe how to handled translation of *.appdata.xml files, lacking rules for newer *.metainfo.xml files.
For handling metainfo files, we currently need to have a build dependency on appstream-glib, which in turn pulls in gtk+:3 and so on, just because we are missing a couple lines from the gettext appdata.loc file, and perhaps in some cases from appdata.its file.
To see what I mean, install appstream-glib and compare the following files with eachother:
diff -u --color=auto /usr/share/gettext-0.19.8/its/appdata.loc /usr/share/gettext/its/appdata.loc
diff -u --color=auto /usr/share/gettext-0.19.8/its/appdata.its /usr/share/gettext/its/appdata.its
If gettext version of them would be updated (I suspect their version control has it - they just haven't released for years...), we'd stop needing to add an explicit appstream-glib build dep on various packages just for the .loc data file and just depend on that newer gettext revision instead (we need to depend on some gettext version anyways) - especially with meson based builds.
As-is, we need to add appstream-glib build depend if it has any metainfo.xml files in translation (i18n.merge_file), but not appdata.xml (as gettext ships rules for it) UNLESS developer_name tag is used, which doesn't have a rule in current gettext appdata.its..
Having this handled in gettext would be much preferable to having to split out the two tiny data files from appstream-glib into some appstream-glib-data package, especially if some upcoming gettext release would update its own copies anyways.
For more context:
Here's the relevant upstream change, although as Mart pointed out in IRC, it seems to be incomplete:
Some additional tags in the ITS file of newer appstream-glib: