Thanks Malaiac.
In the meantime, I added support for woocommerce products.
For other people reading this, who need woocommerce support:
The following will add support for woocommerce products. Of course this will be overwritten after a plugin update but I had to find a quick solution.
In the file classes/deepl-metabox.php
In class WPDeepL_Metabox {
Instead of:
'screen' => array('post', 'page'),
change it to:
'screen' => array('post', 'page', 'product'),
