Thanks for reaching out. I’m not too familiar with WPML and I don’t know if it has any feature that would allow you to translate it. However, the digests plugin supports templating, so you should be able to create a translated digests message.
To do so, just copy the tmpl.php files found in plugins/bbpnns-digest/templates into wp-content/themes/YOUR_CHILD_THEME/bbpnns-digest and modify them as needed. Just be careful not to touch any of the actual PHP parts as that will break the email.