dropdown = function(e){
    var obj = $(e+'.dropdown');
    var btn = obj.find('.btn-selector');
    var dd = obj.find('ul');
    var opt = dd.find('li');

    obj.on("mouseenter", function() {
        dd.show();
    }).on("mouseleave", function() {
        dd.hide();
    })

    opt.on("click", function() {
        dd.hide();
        var txt = $(this).text();
        opt.removeClass("active");
        $(this).addClass("active");
        btn.text(txt);
    });
}

dropdown('#lang-selector');

<div id="lang-selector" class="dropdown">
    <a href="index.php?lang=nl" class="btn-selector">NL</a>
    <ul>
        <li><a href="index.php?lang=en">EN</a></li>
        <li><a href="index.php?lang=nl">NL</a></li>
    </ul>
</div>

// custom theme
var CustomTheme = function () {

    var _initInstances = function () {
        if ($('.vk-home-dark').length) {
            $('#scrollUp').addClass('inverse');
        }

    };

    return {
        init: function () {
            _initInstances();
        }
    };
}();

$(document).ready(function(){
    PreLoader.init();
    CustomTheme.init();
    MediaPlayer.init();
});