Display 1 Post per taxonomy only gives 1 Language

<?php get_header() ?>

<div class="row" id="main">
    <div class="large-12 columns">
        <ul class="small-block-grid-3 large-block-grid-6">
        <?php

        $lang = pll_current_language();

        $terms = get_terms( 'kunden' );

        foreach( $terms as $term ) :

            $args = array(
                'post_type' => 'portfolio',
                'kunden' => $term->slug,
                'posts_per_page' => 1,
                'lang' => $lang,
            );
            $single = new WP_Query( $args ); ?>
            <?php if ($single->have_posts()) { ?>

            <?php while ($single->have_posts()) : $single->the_post(); ?>

            <?php get_template_part('project-thumb'); ?>

            <?php endwhile; ?>

            <?php } ?>
        <?php endforeach; ?>
        </ul>
    </div>


</div>


<?php get_footer() ?>