systemd depends on the gettext package for multilingual support.
You'll find the i18n files in the po/ directory.
The build system (meson/ninja) can be used to generate a template (*.pot), which can be used to create new translations.
It can also merge the template into the existing translations (*.po), to pick up new strings in need of translation.
Finally, it is able to compile the translations (to *.gmo files), so that they can be used by systemd software. (This step is also useful to confirm the syntax of the *.po files is correct.)
To create a translation to a language not yet available, start by creating the initial template:
$ ninja -C build/ systemd-pot
This will generate file po/systemd.pot in the source tree.
Then simply copy it to a new ${lang_code}.po file, where ${lang_code} is the two-letter code for a language (possibly followed by a two-letter uppercase country code), according to the ISO 639 standard.
In short:
Then edit the new po/${lang_code}.po file (for example, using the poedit GUI editor.)
Start by updating the *.po files from the latest template:
$ ninja -C build/ systemd-update-po
This will touch all the *.po files, so you‘ll want to pay attention when creating a git commit from this change, to only include the one translation you’re actually updating.
Edit the *.po file, looking for empty translations and translations marked as “fuzzy” (which means the merger found a similar message that needs to be reviewed as it's expected not to match exactly.)
You can use any text editor to update the *.po files, but a good choice is the poedit editor, a graphical application specifically designed for this purpose.
Once you're done, create a git commit for the update of the po/*.po file you touched. Remember to undo the changes to the other *.po files (for instance, using git checkout -- po/ after you commit the changes you do want to keep.)
You can recompile the *.po files using the following command:
$ ninja -C build/ systemd-gmo
The resulting files will be saved in the build/po/ directory.