This is a companion project to Firefox Relay Website project. The sync between localized and the core repo is once a day. To test your localization, you need to download the build at https://github.com/mozilla/fx-private-relay-add-on/releases. Click on the Assets arrow, and you should see three files. Install firefox_relay-2021.8.31.885-an+fx.xpi, then download the appropriate compressed file to your machine. The build is manual and is done every few days for the time being.