It is up to the WS handler to configure task instance that is created and by default it creates all the I18N elements with en-UK that's why you see that entry only. So if you would like to change the default you would need to extend the current work item handler for human task. Obviously that needs to be enhanced so if you don't mind please create a jira for this so handlers can be enhanced to cover configurable I18N tasks data.
HTH