If you want your Event Manager occurrence to be for International Events, then you can request for your occurrence to have that option enabled. This will add a 'Nationality' field to your Booking form and also give you the option to enable the 'Country' field for addresses.
The Country Selector is an option that can be turned on in the Event Manager occurrence, however, the occurrence must be an international one before you can do this. Simply go into 'Setup', then 'Form' and then finally click on the tab 'Find An Event'.
You should see the tick box within that tab. Once you tick it, there should be a country filter on the form as seen below.
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