When working within the ZMI, you have opportunities to make mistakes that have rippling effects within your Plone site. However, as with all Zope and Plone actions, any settings that you modify or template that you add can be removed using the omnipresent Undo action. For example, if you are modifying the location of slots and, while cutting and pasting, accidentally enter the name of the slot twice, you can simply undo the action and try again. Similarly, if you accidentally change the visibility of slots via the ZMI and find when you log in to Plone that all your slots are gone, you can simply return to the ZMI and either undo the action or just try again. The ZMI is designed to make it difficult for a manager to lose data or settings because previous versions are only an Undo action away.
As you're working with Plone add-ons such as PloneLanguageTool, it's a good idea to stop and start Zope after installing the product. This way, if you have an error when Zope restarts, you can pinpoint it to the item recently added or the setting recently modified. At the very least, if you are logged in to your Plone site as a member, log out and log back in after the new item has been installed and configured. Doing so provides an opportunity to start with a clean slate as both a manager (in the ZMI) and a member (in the Plone site); you can be sure that you have access to the current add-ons and settings of your application.