Ok, found it out:
<ul class="nav navbar-nav">
<li<?php if ( is_category('app-functies')) { echo ' class="current"'; } elseif ( is_singular( 'augmented-reality' )) { echo ' class="current"'; } ?>><a href="https://wordpress.stackexchange.com/questions/112125/<?php echo site_url(); ?>/app-functies/">App Functies</a></li>
</ul>
If you want to add more then 1 is_category() you can also use is category('') || is_singular('') .
Basic PHP knowledge.. but not for a designer like me. Hope this help a lot for other people as well.