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I don’t have the plugin to test this, but does it work if you changed this code in wp-content\plugins\expand-divi\inc\features\ExpandDiviRelatedPosts.php:

$html.= '<h2>' . esc_html( $related_posts_title ) . '</h2>';

To:

$html.= '<h2>' . __( esc_html( $related_posts_title ), 'expand-divi' ) . '</h2>';

If I make changes on the php file that would change it for all languages, I only want to change it for Dutch and leave it for English.

This is how WPML recommends adding dynamic strings, after doing that you should be able to detect the string with WPML and translate it.