$(function () {

    $("#dinner-reservation").submit(function () {
        var valid = true;

        $('input:visible', this).each(function () {
            if ($(this).val() === '') {
                valid = false;
                return;
            }
        });

        if (valid === false) {
            if ($('html').attr('lang') === 'sk') {
                alert('Nevyplnili ste všetky údaje!');
            } else {
                alert('You have not filled in all the necessary information!');
            }
            return false;
        }
    });

});