Comment 1 for bug 538851

After a conversation with pitti, it seems that for packages using python-distutils-extra there is no easy way of adding the X-Ubuntu-Gettext-Domain key. The langpack.mk rule assumes there is a po/Makefile file where to obtain the domain name from, which is not the case with apps using the p-d-e build system. Here's an extract of the conversation on #ubuntu-desktop:

<pitti> dpm: hmm -- the current build system doesn't even know about which gettext domain it's using
<pitti> dpm: if we know, or would assume that it's the same as the project name, we might just create a po/Makefile with GETTEXT_PACKAGE=$projectname in python-mkdebian perhaps

The best approach seems to be for now to add the X-Ubuntu-GNOME-Domain key to the upstream sources. This will have the benefit that it will work both for Ubuntu and for OpenSUSE. That's what the linked branch does.