Install the Chosen component from Bootstrap4C.
Add the class .form-control-chosen to your select, and add the following validation to your javascript:
$('form').on('submit', function(){
$('.form-control-chosen[required]').each(function(){
if ( $( this ).val() == '' ) $( this ).next().removeClass('is-valid').addClass('is-invalid');
else $( this ).next().removeClass('is-invalid').addClass('is-valid');
});
});
Thanks to richardwo!