Languages tab in your Configuration > System > Languages menu allows you to:
Activating/deactivating language files is used when you want to disable specific language in your LiveAgent without actually deleting the language file permanently.
To activate/deactivate language file simply click on the toggle button in the column Actions.
To set one of your active languages as default language in your LiveAgent software simply click on Set as default (finger) button in the Actions column.
Only imported and active languages can be set as default. You also can not have more than one language set as default at the same time. Default language is the language that is selected by default on the agent panel login screen.
To edit language metadata and translations of your language file simply click on the Edit (pencil) button in the Actions column.
How to add or edit specific language translations is explained in separate guide here.
To delete language files from your LiveAgent simply click Delete language button and you are done.