It's a place where we can translate levels (guides, comments etc) and components. Here is the [link][(http://direct.codecombat.com/i18n).
And will adding the i18n to the level components in the level editor allow things to be seen/unseen in it?
Yep. We have "Populate i18n" button for that. Usually we do this when a level is complete and reviewed. Also, sometimes we forgot about it. However, we have "tasks" so we can check it.