It means that you broke the translation file and because of that joomla is not able to load it anymore.
Go in the configuration and in the languages tab, edit your translation file, load the latest version from the server and save.
That will reset the translation file text to the default text and fix your problem.
As we explain in the documentation, instead of editing the translation file directly, copy/paste the translation strings in the override area below. That way you won't loose your modifications when you update and you won't break all the translations with your modifications.