By now i edited loop-achievements.php like
<table class="dpa-archive-achievements">
<caption class="screen-reader-text"><?php _e( 'All of the available achievements with the name, avatar, and karma points for each.', 'dpa' ); ?></caption>
<thead>
<tr>
<th id="dpa-archive-achievements-image" scope="col"><?php _ex( 'Bild', 'column header for list of achievements', 'dpa' ); ?></th>
<th id="dpa-archive-achievements-name" scope="col"><?php _ex( 'Achievement Name', 'column header for list of achievements', 'dpa' ); ?></th>
<th id="dpa-archive-achievements-karma" scope="col"><?php _ex( 'Karma', 'column header for list of achievements', 'dpa' ); ?></th>
<th id="dpa-archive-achievements-excerpt" scope="col"><?php _ex( 'Beschreibung', 'column header for list of achievements', 'dpa' ); ?></th>
</tr>
</thead>
<tfoot>
<tr>
<th scope="col"><?php _ex( 'Achievement Image', 'column header for list of achievements', 'dpa' ); ?></th>
<th scope="col"><?php _ex( 'Achievement Name', 'column header for list of achievements', 'dpa' ); ?></th>
<th scope="col"><?php _ex( 'Karma', 'column header for list of achievements', 'dpa' ); ?></th>
<th scope="col"><?php _ex( 'Beschreibungn', 'column header for list of achievements', 'dpa' ); ?></th>
</tr>
</tfoot>
and the single-achievements.php like
<td headers="dpa-archive-achievements-image">
<?php do_action( 'dpa_template_before_achievement_image' ); ?>
<?php dpa_achievement_image();?>
<?php do_action( 'dpa_template_after_achievement_image' ); ?>
</td>
Next question would be, where I can rename the table headers ??