Hi Marco,
To translate the custom fields in the settings page, please create the file wpml-config.xml
in your theme/child theme folder, then add this code to that file
<wpml-config>
<admin-texts>
<key name='option_name'>
<key name='field_id' />
</key>
</admin-texts>
</wpml-config>
Translate the field value in WPML > String translation, show the field value by using the WordPress function get_option()
<?php
$options = get_option( 'option_name' );
echo $options['field_id'];
?>
See my screen record https://share.getcloudapp.com/QwuA02PY