A screen in a touchpad can be useful because it can help users to understand how to use different features like side scrolling or drag-and-drop. Also it can be used to display some useless information, like time, current keyboard layout or CPU load.

Putting buttons on a touchpad is not the best idea because they would interfere with its normal usage.

And by the way, why nobody makes separate keys for layout switching? It is very inconvenient to have a single key to switch layouts (like Win + Space that is the default for Gnome). It would be much better to have separate physical keys for each language so I can start typing with one of them and never type in the wrong layout.

Seriously, why didn't anyone invented it yet?