Your best option is any of the field scripts that trigger when an option is selected from the drop down. My favorite is the Keystroke script on the "Format" tab, when the "Custom" option is selected.
For scripting purposes it is best to have a direct relationship between the options in the dropdown and the OCGs (Layers). I noticed that the OCGs on the PDF are named "Layer 1", etc. These names make the scripting more complicated, because you need to somehow relate "Layer 2" to "USA".
For example, have the OCGs named for the country, so the OCG name and the dropdown name are the same, i.e. the layer for USA is named "USA". Or have them in the same order so the index if the dropdown selection is the same as the position of the the OCG in the returned array. I don't like this one cause it's hard to manage. Here's an example that assumes the names are the same.
In Format Keystroke script
if(event.willCommit)
this.getOCGs().some(function(a){return a.state = (a.name==event.value);});
To use the code you'll need to do two things,
1. Rename all layers to their country names
2. Flatten out "Layer 1", since it is the main background for the document.
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