I've managed to replace all of the gettext and intltool external dependency and replace it with the python babel library instead. It's actually much simpler as well, which is nice.

Notes:

To generate the po/terminator.pot you can now just run python setup.py extract_messages, I've replaced the contents of po/genpot.sh with the correct command
It uses the same mechanism to generate the .mo files tied into the same python setup.py build command.

The old build command would run a tool called "intltool-merge" that would merge all of the translated strings into the data/terminator.desktop and data/terminator.appdata.xml. The new version just pulls the translations out and uses the same localization machanisms as the python files

Notes for distributors: If your build environment is not able to run pip install or is otherwise detached from the net, you will need to add the babel library package explicitly to the BuildRequires section of the respective package. In fedora it's python3-babel.

I've run a few tests and make sure it all works but I would appreciate it if other people would take a look too. @lazyfrosch can merge it once he thinks it's ready.