This simple docker-compose.phpmyadmin.yaml just mounts a file from the host into the dba container.
You can create a directory in the .ddev folder named phpmyadmin. In there you can create a custom settings file config.user.inc.php.
When you restart ddev, the settings file will be used when starting phpmyadmin.
Example file contents of config.user.inc.php:
<?php
$cfg['Lang'] = 'nl';
$cfg['DefaultLang'] = 'nl';An overview of all the possible settings can be found in the phpmyadmin manual.