This property is offered for each Environment of the Knowledge Base, but only if the Environment Translation Type Property was previously set to Static.
It allows setting the language the developer wants to translate the Knowledge Base texts into (like label texts, or texts inside quotes in rules, sources, etc.).
One of the selected language(s) in the Localization node (under Customization) of the KB Explorer:
The offered languages to be selected are predefined, or the developer can create a new Language object.