First off, thank you for considering translating gwion.
Thanks to the build system, you're gonna get on tracks fast.
Make your live easieruse
export TRANSLATION_TARGET=xxxso you don't have to repeat it on the command line
You have to make sure there is a directory for your target language (e.g.: fr, en, es_ES ...).
make translation-init TRANSLATION_TARGET=<xxx>
<xxx> is your language of choice.
It will fail if your language already exists, but this is not a problem.
Next, you should adjust the translations. What about using your favorite editor?
make translation-edit TRANSLATION_TARGET=<xxx>
Maybe the sources changed, and there is more messages to translate?
This will update all languages.
You can now get back to step 2.
It's now time to add your changes to the package
make translation-commit TRANSLATION_TARGET=<xxx>
Now please submit a pull request.