Translations are done on Launchpad at https://translations.launchpad.net/linuxmint/latest/.
From Launchpad we export two sets of files:
- The mo files go to files/usr/share/locale/ and eventually end up in the Cinnamon package itself
- The po files go to po/po-files/. They are not compiled into mo files (this is done by Launchpad) but are present in this archive as source for the mo files.