I realize that I am a bit late to the party here, but I am wondering why #2 and #3 change the name of the translation files to become specific to the libexif-gtk gtk2 and gtk3 versions?
I also do not understand why libexif-gtk does not build both the gtk2 and the gtk3 version from a single source tree build.
PKG_CHECK_MODULES([GTK2], [gtk+-2.0 >= 2.4])
PKG_CHECK_MODULES([GTK3], [gtk+-3.0])
and then later use GTK2_LIBS for libexif_gtk_LIBADD and GTK3_LIBS for libexif_gtk3_LIBADD.
Why can't both libexif-gtk.so.* (gtk2) and libexif-gtk3.so.* (gtk3) use the same file /usr/share/locale/de/LC_MESSAGES/libexif-gtk5.mo? Then the package with libexif-gtk and libexif-gtk3 can both depend on the same (possibly even noarch) -translations subpackage.
Just renaming the translation files does not change their content, after all, so I do not see what renaming and duplicating the files buys us.
(Also, the name of the *.pot and *.po files changing might confuse translators and their toolchains.)