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!