Это можно сделать достаточно легко, если вы добавите класс в поля категории страны, а затем соответствующим образом измените строку-разделитель.
$(document).ready(function() {
$("input[name='A']").change(function() {
var checkedCountries = $("input[name='A']:checked");
var countries = [];
for (var i = 0; i < checkedCountries.length; i++) {
// default separator
let separator = ", ";
// no comma after category
if ($(checkedCountries[i]).is('.category')) {
// EDIT add line break after category
//separator = " ";
separator = "\n";
}
if (checkedCountries[i + 1] == undefined) {
// last item needs no separator
separator = "";
} else if ($(checkedCountries[i + 1]).is('.category')) {
// the next item is a category, separate with new line
separator = "\n";
}
// put separator at the end of the value
countries.push(checkedCountries[i].value + separator);
}
// join with null
$("#countryList").val(countries.join(""));
});
$("#copyBtn").click(function() {
var copyText = document.getElementById("countryList");
copyText.select();
document.execCommand("Copy");
alert("Copied the text: " + copyText.value);
});
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="mainpage_box">
<h3><input type="checkbox" name="A" value="COUNTRY 1:" class="category"><span>COUNTRY 1:</span></h3>
<div id="mainpage-admins" title="System administrators" class="items">
<input type="checkbox" name="A" value="America">America<br>
<input type="checkbox" name="A" value="Burma">Burma<br>
<input type="checkbox" name="A" value="China">China<br>
<input type="checkbox" name="A" value="Denmark">Denmark<br>
<input type="checkbox" name="A" value="England">England<br>
<input type="checkbox" name="A" value="France">France<br>
</div>
</div>
<div class="mainpage_box">
<h3><input type="checkbox" name="A" value="COUNTRY 2:" class="category"><span>COUNTRY 2:</span></h3>
<div id="mainpage-admins" title="System administrators" class="items">
<input type="checkbox" name="A" value="America">America<br>
<input type="checkbox" name="A" value="Burma">Burma<br>
<input type="checkbox" name="A" value="China">China<br>
<input type="checkbox" name="A" value="Denmark">Denmark<br>
<input type="checkbox" name="A" value="England">England<br>
<input type="checkbox" name="A" value="France">France<br>
</div>
</div>
<textarea id="countryList" rows="20" cols="150"></textarea></br>