For your information, I opened an HI ticket.
Here's the answer, you have to upgrade to the Orlando version.

Upon careful analysis of your query, we found that this is the expected behaviour of the release that Dynamic Translation for comments & work notes is a new feature released as part of Orlando release.

You are currently in the NewYork version. I would suggest you upgrade to Orlando to achieve the functionality.

Please find the below Orlando Release notes for the information:
https://docs.servicenow.com/bundle/orlando-release-notes/page/release-notes/summary/rn-summary-new-features.html
https://docs.servicenow.com/bundle/orlando-platform-administration/page/administer/dynamic-translation/concept/dynamic-translation-forms.html