If your text service wishes to display some UI on the language bar, then your service will need to create objects that implement one (or more) of the following interfaces:
Pass this base interface to ITfLangBarItemMgr::AddItem
Derived from ITfLangBarItem
If your text service has an object that implements ITfLangBarItem (or one of its subclasses), and that object updates the item, that object must also implement ITfSource (and call ITfLangBarItemSink::OnUpdate) so that TSF can redraw the language bar correctly. See the article in MSDN on Implementing a Language Bar Object, and the sample source code.