The first step is to create the initial drop-down box. This is a shape in Storyline. On this shape create a normal state with text ‘ select your language’  and a down arrow. Nog create a custom state for each item in your drop-down list. These states will be used to display the selection that is made.

Next, create a new slide layer to display the the list items when the drop-down is expanded.

To show what selection is made by the user we need to set a trigger on each list item that changes the state of the original drop-down box to the state that matches the selected item.

Finally, you’ll need a second trigger on each list item that hides the current layer containing the list.