The Country column represents the data for which you want to create the pie chart (with percentage for each country). You need to add a column and fill it with 1 (this will be used to count how many times each country appears in my Country column).
Select the data from both columns and create a Pivot Table: go to Insert tab and click on the Pivot Table | PivotTable button. You have the option to create the pivot table in the same worksheet or on a new sheet)
On the PivotTable Field List drag Country to Row Labels and Count to Values.
Now select the pivot table data create the pie chart:
Using pivot tables allows you to update the data in the pie chart very easily – all you need is to update the Country column.