If you are trying to localize the property values within the UI, you need to do something else. The UI will use the language the UI is set at and not the language the site is selected to use. Meaning you'd have to change the Kentico UI to bg-BG in order for the dropdown to use the correct value. To get by this you can use this macro:
{% GetResourceString("your.custom.key", QueryString.culture) |(identity)GlobalAdministrator%}
The LocalizationContext
object is only available on the live site and not in the UI.