Update-Notifier must get new translation, all old files are lost...
You can use bash_trans to create a pot file of update-notifier. Open /usr/share/apps/update-notifier/ and try a right click on update-notifier, and update-notifier-launch. Use Translation Bash scripts, next step is, open it with poedit and translate it...
Good luck
ps.: update-notifier works here so far well...