Steps to add a translation
Here are the steps you can follow to add or update the translation for your language. Please feel free to ask on the #gnome-shell IRC channel on irc.gnome.org if you need additional help.
Follow these instructions to build GNOME Shell with jhbuild.
Go to the GNOME Shell translation page
If the file for your language already exists, click on your language name and download the 'Translated' file which will contain up-to-date strings from GNOME Shell. Replace the po/<your lang>.po file by the downloaded file.
If you are creating a new language file, download the "POT file" available on the aforementioned page and save it under po/<your lang>.po.
Add the translation text. You can look at gnome-menus and gnome-panel module translations for your language to see how some of the strings that are used in gnome-shell have been translated in the past.
Run jhbuild buildone -f -n gnome-shell to build the shell with your changes and then restart it by typing restart in the Alt+F2 dialog to check your changes.
Once you are happy with your changes, send your updated po file on the appropriate language page from the GNOME Shell translation page. Someone from your language team should review and commit the file for you.
What if a string is not showing up for translation?
It's possible that the file containing the string has not been added to POTFILES.in or that the string has not been properly initialized with the gettext utility. You can file a bug in the GNOME Shell Bugzilla if you see a problem like that.