Loop through custom fields with similar label / common chars in label

You display data in loop, so all item have the same ID div id="demo". Add some suffix to make them unique. For example:

$lang = substr($field['name'], strpos($field['name'], "_");

echo '<button data-toggle="collapse" data-target="#demo-'. $lang .'"> ....';

echo '<div id="demo-'. $lang .'" class="collapse"> ...</div>'

Edit

Audio doesn’t work inside the collapse panel because key $name never meets both conditions:

if (stripos($name, 'isbn') !== false): 
   if (stripos($name, 'hoerprobe') !== false)

$name is either hoerprobe_arabisch or isbn_arabisch.

Try this code:

if (stripos($name, 'isbn') !== false) : ?>
   <?php $lang = substr($field['name'], strpos($field['name'], "_")); ?>
   <button data-toggle="collapse" data-target="#demo<?php echo $lang ?>">
      Deutsch/<?php echo substr($field['label'], strpos($field['label'], " ") + 1); ?> </button>

    <div id="demo<?php echo $lang ?>" class="collapse">
         Lorem ipsum dolor text....

       <?php //isbn
          echo $field['prepend'] . " " . $field['value']; ?> </br>

       <?php //titel
          echo the_field('titel'. $lang); ?> </br>

       <?php //coverimage
          $titelbild = get_field('titelbild' . $lang); ?>
          <?php if ($titelbild) : ?>
              <img src="https://wordpress.stackexchange.com/questions/309536/<?php echo $titelbild["url']; ?>" alt="https://wordpress.stackexchange.com/questions/309536/<?php echo $titelbild["alt']; ?>"/>
          <?php endif; ?></br>

       if (isset($fields['hoerprobe_'.$lang])) : ?>

           <?php
           $hoerprobe_field_name="hoerprobe".$lang;
           $hoerprobe_field = $fields[$hoerprobe_field_name ];
           echo $hoerprobe_field['label'];
           $file = get_field(hoerprobe_field_name); ?>

           <audio controls>
               <source src="https://wordpress.stackexchange.com/questions/309536/<?php echo $file["url']; ?>">
                    Your browser does not support the audio tag.
               </audio></br>
       <?php endif;

    </div>
<?php endif;