When I install the Geta.optimizely.categories nuget package on a new site, then go to edit a category, I see "IsSelected" as one of the properties. Notice that there is no space between "Is" and "Selected". That is because the language xml file is not being used.
When I tested this using the sandbox Foundation, it did include the space. I saw that the sandbox includes a /lang/ geta.epicategories.xml However most epi sites will not have that. It's not installed by the nuget installer.