I found that the text strings for the email subject, like "Comment in a topic that you moderate has been unpublished", are placed in administrator/language/en-GB/en-GB.com_comment.ini
For consistency reasons it would be better to have them in language/en-GB/en-GB.com_comment.ini.
I don't know if this is possible because of the structure of your great component.
To avoid getting overridden by updates, I placed the relevant language strings in administrator/language/overrides/en-GB/en-GB.override.ini and german translations in administrator/language/overrides/en-GB/de-DE.override.ini
Email subject text translations
5 years 2 weeks ago #21062
I'm not 100% sure why I put them in the administrator part. Maybe because emails are sent also in the backend (whenn you approve a comment or so). But it actually doesn't really matter -> both admin and frontend files are merged when in frontend.