Customize Wordpress Site With Query

I want to show different contents to readers from different countries in my WordPress site. I don't want to be installing different WordPress for different country. Now the thing is, I want to filter contents targeted at a particular country to readers from the country. There will be a field (checkbox field) attached to each post that I will be able to select countries or country a post is targeting. And on the homepage, in the header, I will list different countries (like dropdown menu) for the user to select their country and when a user select France, it will show specific contents to France by filtering the keyword 'France'. The algorithm will be like this. 1. A field that will take multiple countries will be in the post area when creating a post so I can input the country I'm targeting with each post. [This has been done already. I used a plugin called Advanced Custom Field to handle this. Check the attached image for how it looks]. 2. A drop-down on the home page of the site where a user can select his or her country manually. When a user selects South Africa, every post on the homepage will change to posts targeting South Africa. So the url will be like this [login to view URL], where country is the name of the field and ZAR is the value. What you will need to do is to write a code that will query the database based on the url parameter. It will filter the value submitted in the url via GET method.